Leonard

Johnny Galecki

Leonard Leakey Hofstadter^[1] (portrayed by Johnny Galecki; seasons 1-12) is an experimental physicist with an IQ of 173. Originally from New Jersey, he received his PhD at age 24, spending at least some of his time at Princeton University. He shared an apartment in Pasadena with colleague and friend Dr. Sheldon Cooper for much of the series. Unlike his equally geeky friends, Leonard is interested and adept in building relationships with other people. Compared to his friends, he is relatively successful with women. Leonard comes from an accomplished family. His mother, Dr. Beverly Hofstadter, is a neuroscientist and world-renowned psychiatrist with whom he has a strained relationship. His sister is a medical researcher, his father, Dr. Alfred Hofstadter, is an anthropologist, and his brother Michael is a Harvard law professor. He and Penny date on and off throughout the series, eventually marrying and finding out that Penny is pregnant in the series finale.

Leonard makes a brief non-speaking cameo as a child in the Season 2 finale of Young Sheldon; he was portrayed by Isaac Harger as a child.

Sheldon

Jim Parsons

Sheldon Lee Cooper (portrayed by Jim Parsons; seasons 1-12) is a theoretical physicist, possessing a B.S, M.S, M.A,^[2] Ph.D, Sc.D, I.Q. of 187, and eidetic memory. Born in East Texas, he was a child prodigy and received his PhD at age 16 from the California Institute of Technology. He is a cold, condescending, self-centered, and immature person but is rarely malicious. He is obsessed with routines and any disruptions distress him enormously. He shows signs of obsessive–compulsive personality disorder, often shown through his compulsive need to knock three times on a door while saying the name of the person behind the door three times too and the superiority complex. Despite his lack of social graces, he does care about his friends and family. In seasons 1–9, he shares an apartment in Pasadena with friend and fellow physicist Dr. Leonard Hofstadter. From then after, he moves in with his girlfriend and eventual wife, Amy. It is revealed in Young Sheldon that they have kids too.

His interest in science fiction and comic books borders on the extreme. He has a particular affinity for Spock from the Star Trek franchise. Although he has the tendency to take things literally, he is fond of occasionally telling a joke or playing a prank, which he typically punctuates with his trademark exclamation, "Bazinga!"

Penny

Kaley Cuoco

Kindhearted and outgoing, Penny (portrayed by Kaley Cuoco; seasons 1-12) is Leonard and Sheldon's neighbor across the hallway from the inception of the series, replacing the apartment's former occupant, Louis/Louise, a "transvestite with a skin condition" according to Sheldon. Originally from a small town outside Omaha, Nebraska, she was a waitress and occasional bartender at the local Cheesecake Factory until season seven, and is an aspiring actress. Sheldon describes her as a "good-natured simpleton" and is critical of her dropping out of community college.

Initially, not much is known about Penny's family, but it is mentioned in the series that her father, Wyatt (portrayed by Keith Carradine,^[3]) raised her like a boy, her mother smoked marijuana while she was pregnant with her, her sister shot her husband while they were intoxicated, and her brother is a meth dealer. Her mother, Susan (Katey Sagal), and brother, Randall (Jack McBrayer), are finally seen in the first episode of season ten. Her last name is never revealed during the series. Leonard and Penny date on and off throughout the series which is a big plot point of the show, they eventually marry and find out they're pregnant in the series finale.

Penny makes a brief non-speaking cameo as a child in the Season 2 finale of Young Sheldon, she was portrayed by Quinn Aune as a child. Cuoco additionally voices the pool water, part of one of young Sheldon's nightmares, in the spinoff.

Howard

Simon Helberg

Howard Joel^[4] Wolowitz, M.Eng. (portrayed by Simon Helberg; seasons 1-12) is an aerospace engineer at Caltech's Department of Applied Physics who often hangs out at Leonard and Sheldon's apartment. Unlike Sheldon, Leonard, and Raj, Howard does not have a doctorate and often gets disparaged as a result, especially by Sheldon. He defends this by pointing out that he has a master's degree in Engineering from MIT and that the equipment he designs is launched into space, unlike the theoretical work of his friends. In the fifth-season finale, Howard goes to the International Space Station on Expedition 31.

Howard lives in Altadena with his domineering, belittling, and unseen mother, who treats him like a child. While he sometimes expresses irritation at this treatment, for the most part, he appears to prefer it.^[5] Howard and his mother often communicate with each other from different rooms by yelling, a habit which Bernadette also adopts in later episodes. Howard fancies himself a ladies' man and attempts pick-up lines whenever a woman is present, although he drops this habit once he starts going out with Bernadette. He is a non-observant Jew. Over the course of the series, he and Bernadette marry and have two children.

In the pilot episode, he speaks English, French, Mandarin, Russian, Arabic, and Persian, along with fictional languages like Klingon and Sindarin. Howard suffers from asthma, transient idiopathic arrhythmia, allergies to peanuts, almonds, and walnuts, and is prone to canker sores and pink eye.

Howard makes a brief non-speaking appearance in the Season 2 finale of Young Sheldon, in which he was portrayed by Ethan Stern as a child; he returns to reprise himself in a narration with Helberg's voiceover for a Season 5 episode.

Raj

Kunal Nayyar

Rajesh Ramayan^[6][7] Koothrappali, PhD (portrayed by Kunal Nayyar; seasons 1-12) is Howard Wolowitz's best friend. Often called "Raj" by his friends, he is from New Delhi, India, and works in the physics department at Caltech, researching astroparticle physics. Raj comes from a very wealthy family in India, and often communicates with his parents, Dr. and Mrs. Koothrappali, via webcam. He did at least part of his higher education at the University of Cambridge and like most of the scientists bar Howard he also has a PhD.^[8] He has a younger sister, Priya. He lives in an apartment in Pasadena. As with his friends, he is mutually involved with and obsessed with science fiction and comic books in general. He is also a fan of Harry Potter and Indian music, but appreciates the Indian lullabies his mother sang and the catchiness of Hindi phrases. Unlike his male friends, Raj has many feminine interests, such as reading Archie comics and Twilight and watching chick flicks such as Bridget Jones's Diary. It is speculated that Raj might be gay due to his feminine interests and his close friendship with Howard, with whom he has arguments similar to those of a married couple. However, Raj has always stated that he is straight, but metrosexual. He was infatuated with Penny, and secretly wrote love poems about Bernadette.

Raj is a Hindu and believes in karma (reincarnation), but eats beef. He is very shy around women outside of his family, and during the first six seasons of the show, found himself unable to speak to women while in their presence unless he drank alcoholic beverages, or believed he had done so.^[8] He eventually is able to get over this condition and starts to, unsuccessfully and for short periods, date women.

Raj makes a brief non-speaking cameo as a child in the Season 2 finale of Young Sheldon, he was portrayed by Rishabh Prabhat as a child.

Leslie

Sara Gilbert

Leslie Winkle, PhD (portrayed by Sara Gilbert; main: season 2; recurring: season 1; guest: seasons 3 and 9) is an experimental physicist who shares her lab with Leonard. In appearance, she is essentially Leonard's female counterpart, equipped with black-framed glasses and sweat jackets. She suffers from lazy eye. She is one of Sheldon's archenemies due to their conflicting scientific ideas and takes every opportunity to insult Sheldon for mistreating her.

Bernadette

Melissa Rauch

Bernadette Maryann^[9] Rostenkowski-Wolowitz,^[10] PhD (portrayed by Melissa Rauch; main: seasons 4-12; recurring: season 3) is originally a waitress and co-worker of Penny's at The Cheesecake Factory, using her wages to pay for her graduate studies in microbiology. She is a smart, short-tempered, ruthless, competitive young woman. Despite her short stature and squeaky voice, she is regarded as being intimidating, largely because of her sharp tongue, aggressive demeanor and tendency to be a bully. A recurring joke in the series references Bernadette and her lab team handling dangerous or infectious specimens, leading to accidental byproducts.

While initially not getting along with Howard's mother, she eventually becomes like her, domineering and passive aggressive, even adopting her signature yell.

She and Howard have two children, Halley and Neil Michael (who they call Michael), in seasons 10 and 11 respectively.

Bernadette is of Polish origin and Catholic upbringing, and was originally seen wearing a cross necklace.

Bernadette makes a brief non-speaking cameo as a child in the Season 2 finale of Young Sheldon, she was portrayed by A. J. Coggeshall as a child.

Amy

Mayim Bialik

Amy Farrah Fowler, PhD (portrayed by Mayim Bialik; main: seasons 4-12; guest: season 3) is a neuroscientist with a research focus on addiction in primates and invertebrates, occasionally mentioning such experiments involving addiction. (Bialik herself has a PhD in neuroscience.) She is Sheldon's love interest in the series, and they marry at the end of season 11. They are revealed to have children in Young Sheldon.

Early on, Amy is essentially a female counterpart to Sheldon; as the series progresses, she begins a campaign to increase Sheldon's feelings for her by becoming more involved in his interests, including video games and Star Trek, and treating him as his mother did.

In 2020, a year after The Big Bang Theory went off the air, it was revealed that Kate Micucci, who ultimately got the role of Lucy, had auditioned for the role of Amy Farrah Fowler.^[11]

Amy makes two appearances in Young Sheldon. She makes an appearance as a child in the Season 2 finale where she is portrayed in a non-speaking role by Lily Sanfelippo. Bialik returns to play Amy in the narration of the season 4 premiere.

Stuart

Kevin Sussman

Stuart David Bloom^[12][13] (portrayed by Kevin Sussman; main: seasons 6 and 8-12; recurring: seasons 2-5 and 7) runs the Comic Center in Pasadena,^[14] the comic book store that the characters often visit. Stuart is characterized by his low self-esteem and loneliness, which often result in pathetic attempts to engage with women and win favor with the gang. This is despite owning his own business and being a talented portraiture artist who attended the Rhode Island School of Design.^[15]

One of the running gags on the show is Stuart's medical situation. It is depicted as disastrous, so much so that he can be seen under various medical treatments. He is said to have depression and possible malnutrition, among other problems. In the last season, Stuart's life seems to be turning around. His comic store becomes successful and he gets a steady, long-term relationship.

Kevin Sussman was a recurring guest actor from seasons two to five, and was promoted to starring role for the sixth season onwards.^[16]

Emily Sweeney

Emily Sweeney, M.D., (portrayed by Laura Spencer; main: season 9; recurring: seasons 7-8; guest: season 10) is a dermatology resident at Huntington Hospital, whom Raj finds on an online dating site. Raj enlists Amy's help to be his “online wingman” and talk to Emily in his behalf. Amy tells Emily about this and it puts her off. Amy and Emily end up hitting it off, having both gone to Harvard and both being into quilting. While Amy and Emily are at lunch together, Raj crashes the lunch. This drives Emily away from both Raj and Amy this time. Later, Raj and Emily meet again, accidentally and hit it off. They eventually double date with Howard and Bernadette, which horrifies Howard. He and Emily had a horrible date, which ended because Howard had gas station sushi, got food poisoning and clogged and overflowed her toilet. Emily is not afraid to stand up for herself or to others when necessary. She is shown to have a slightly alarming personality trait: she delights in the macabre. Raj ends up breaking up with her before Valentine's Day so he could be with Claire (Alessandra Olivia Toreson). Unfortunately for Raj, Claire had just gotten back together with her boyfriend. So, he eventually lost them both. Emily attempted to get back into Raj's good graces and did manage to sleep with him one last time, but it didn't last.

Laura Spencer was a recurring guest actress on the series in seasons seven and eight. During production of season 9, she was promoted to a "fractional" starring role; the upgraded status ensured her availability on an as-needed basis, without requiring her in every episode.^[17] She returned as a guest in season 10.

Sheldon Lee Cooper

Sheldon Lee Cooper (portrayed by Iain Armitage; seasons 1-present) is a child prodigy who finishes primary school before his twin sister, Missy, and joins the same high-school class as his older brother, Georgie, at the age of nine. He reads widely and possesses an eidetic memory. He prefers to learn topics that interest him on his own—such as Faraday's law of induction and gravitational lensing—than the standard curriculum of grade school. In this series, the origins of his interests in the sciences, engineering, computers, trains, comic books, and science fiction (especially the character Spock from Star Trek) are revealed. Sheldon moves on to college while Missy remains in middle school. Despite their rivalry, Sheldon and Missy are closer than they first appear.

Mary Cooper

Mary Cooper (née Tucker)^[18] (portrayed by Laurie Metcalf in The Big Bang Theory; recurring: seasons 7 and 9–12; guest: seasons 1 and 3–5 and by Zoe Perry in Young Sheldon; seasons 1–present) is Sheldon's short-tempered, but loving and deeply religious mother from Texas. She is able to control Sheldon, with Leonard once describing her to Penny as "Sheldon's kryptonite". A devout Southern Baptist, she also has two other children — Sheldon's twin sister, Missy, and brother, Georgie, who is five years older than the twins. Her husband, a rambunctious alcoholic, was also named George, but he later died in Sheldon's childhood. To Mary's relief, her other children do not share Sheldon's precociousness, even once commenting to Leonard, "I thank the good Lord my other kids are as dumb as soup". Mary herself is not intellectual but is quite wise, though she is prone to making outdated and insensitive remarks. She is nevertheless kind at heart and is tolerant of other faiths. Leonard wishes his mother were as loving as Sheldon's mother, although Sheldon himself appears at best ambivalent about her parenting. Sheldon mentions that Mary once hit him with a Bible because he refused to eat his Brussels sprouts. She apparently thought something was wrong with her son while he was growing up since Sheldon will often remark after someone calls him crazy, "I'm not crazy; my mother had me tested!" Mary confirms this, though she wishes that she had taken him to Houston for further testing.

A younger version of Mary, played by Zoe Perry, is a main character in the prequel series Young Sheldon. Perry is Metcalf's real life daughter.

George Cooper Sr.

George Cooper Sr. is the father of Georgie, Sheldon, and Missy and the husband of Mary Tucker-Cooper. He is the high school football coach for Medford High. In The Big Bang Theory, it is established that he died when Sheldon and Missy were fourteen years old. There is currently no information on how he died, but he has shown heart attack-like symptoms and even wound up in the ER for it. He is portrayed by Lance Barber (seasons 1-present), who previously portrayed Jimmy Speckerman in one episode of The Big Bang Theory (guest: season 5). George works as a football coach.

George "Georgie" Marshall Cooper Jr.

George "Georgie" Cooper Jr. (portrayed by Jerry O'Connell as an adult; recurring: seasons 11-12 and Montana Jordan as a youth; seasons 1-present) is Sheldon and Missy's older brother who resides in Texas and runs a car tire business. Often at odds while growing up, the Cooper brothers later reconcile their differences. George Jr. is a laid-back character who likes to make money and is a known womanizer. By season 12 of The Big Bang Theory, it is revealed that he has been twice married and divorced.

Melissa "Missy" Cooper

Melissa "Missy" Cooper (Courtney Henggeler as an adult; guest: seasons 1 & 7 & 11 and Raegan Revord as a child; seasons 1-present) is not as academically accomplished as her twin brother but displays better social skills. As a child, she was obsessed with Alf. In season three of Young Sheldon, Missy joins the baseball team in the fictional town of Medford. While Sheldon finishes high school and matriculates at university, Missy navigates the ordinary challenges of adolescence. Despite their differences, the twins are pretty close; they are both atheists.

As an adult in The Big Bang Theory, Missy is a tall, attractive woman who promptly catches the attention of Leonard, Howard, and Raj. She occasionally visits Sheldon, whom she calls a "rocket scientist" to her friends, much to his annoyance. She's the only character besides Sheldon who appeared in all episodes of Young Sheldon. She was later married and had children but separated from her husband in Season 11.

Constance "Connie" Tucker ("Meemaw")

June Squibb

Constance "Connie" Tucker^[18] (June Squibb in The Big Bang Theory; guest: season 9 and Annie Potts in Young Sheldon; seasons 1-present) is Mary's mother, George Sr.'s mother-in-law, and maternal grandmother to Georgie, Sheldon, and Missy, who call her "Meemaw". Like Sheldon's other family members, she is also native to Texas and is their next-door neighbor in the town of Medford. Sheldon sees himself as her favorite grandchild, whom she affectionately calls "Moonpie". Sheldon, in turn, is very affectionate toward and protective of her. Meemaw is often mentioned by Sheldon but did not appear on the series until episode 14 of season nine, "The Meemaw Manifestation".

In Young Sheldon, a recurring theme is her significant gambling habit.

Pastor Jeff Difford

Pastor Jeff Difford (portrayed by Matt Hobby; main: seasons 3-present; recurring: seasons 1-2) is the upbeat lead pastor of a local Baptist church in Medford that Sheldon's family attends in Young Sheldon. Like Mary, he, too, sometimes has friction with Sheldon's atheistic side but often recommends Sheldon explore his own ideas logically. Every time Sheldon challenged Pastor Jeff's beliefs, he came up short.

Billy Sparks

Billy Sparks (portrayed by Wyatt McLure; main: seasons 5-present; recurring: seasons 1-4) is a boy who lives with his parents and sister next to the Coopers and Meemaw. He has been mentioned in The Big Bang Theory as one of Sheldon's childhood bullies, although in Young Sheldon, he is rather good-natured and at times is friends with Sheldon but is shown to be extremely dim-witted.

Alex Jensen

Margo Harshman

Alex Jensen (Margo Harshman) is a Caltech graduate student who appears in season six. Sheldon hires her to review his kindergarten and elementary-school notebooks for any possible Nobel Prize-winning research and attend to the tasks that Sheldon perceives as a waste of his own valuable time, such as buying Amy a Valentine's Day gift.

Alex takes an interest in pursuing a relationship with Leonard (despite his involvement with Penny), though he is oblivious to her romantic overtures. After she asks him out on a date, to which Leonard is flattered and Penny is jealous, Sheldon views this as an unacceptable distraction from her job. Alex files a harassment complaint against Sheldon after he attempts to give her a "talking to" involving unthinkingly sexist language and showing her a picture book of sexually diseased genitals. After Sheldon apologizes to Alex for his behavior, he tasks her with taking an online sexual harassment course he was mandated to take by the Human Resources department.

Althea

Althea (Vernee Watson) is a nurse and receptionist at the hospital. She operated the sperm clinic during the original (unaired) Pilot episode when Leonard and Sheldon were considering donating. She was also present when Howard needed assistance in buying time to keep Leonard from returning home to see his surprise party getting set up. In Season 4 when Howard had a 'mishap' with a robot arm, she freed him by shutting off the computer.

Anu

Anu (Rati Gupta) is the woman who Raj's father wants him to marry. She first appears in season 12 and works at a hotel as a concierge.

Arthur Jeffries

Bob Newhart

Arthur Jeffries, or Professor Proton (Bob Newhart), is the star of a fictional in-universe science show that Sheldon and Leonard watched as children. The character is a homage to Mr. Wizard.^[20] After the show was cancelled, Jeffries was not taken seriously as a scientist and resorted to doing children's parties as his persona. When Sheldon asks for his wisdom, Jeffries tells Sheldon to appreciate everything in life, including his friends, and never take it for granted.

For his role as Jeffries, in 2013, Newhart won his first Primetime Emmy Award. Newhart plays the same character on Young Sheldon.

Barry Kripke

John Ross Bowie

Barry Kripke, PhD (John Ross Bowie) is a colleague working in plasma physics who frequently clashes with Sheldon. Kripke has a case of rhotacism in which he pronounces the letters "R" and "L" as "W" in much the same way as Elmer Fudd from Looney Tunes, Jimmy Five from Monica's Gang and Gilda Radner in her "Baba Wawa" sketches.

Bert Kibbler

Brian Posehn

Professor Bertram Kibbler (Brian Posehn) is a geologist at Caltech, who met Amy while she was working there in her own lab in "The Occupation Recalibration". He made various other appearances, including forming a band with Raj and Howard in which he sang a song from the point of view of the boulder seen near the beginning of the movie Raiders of the Lost Ark. After Raj stopped accepting his father's money to become financially independent, he moved into the room above Bert's garage.

Beverly Hofstadter

Christine Baranski

Beverly Hofstadter (Christine Baranski) is Leonard's unloving, emotionally/psychologically abusive and overly analytical mother who works as a neuroscientist, as well as a psychiatrist. She is Sheldon's female equivalent regarding neurotically strict speech patterns, disregard for social conventions, and compulsive attention to detail. The lack of Beverly's maternal feelings and actions toward Leonard has led to him having an obsessive need to please. Mrs. Hofstadter and Mrs. Cooper do not get along.

For her role as Beverly, in both 2009 and 2010, Baranski was nominated for an Emmy Award for Outstanding Guest Actress in a Comedy Series.

Claire

Alessandra Torresani

Claire (Alessandra Torresani) is a writer for a children's science fiction series whom Raj and Howard meet in the comic book store in season 9. Claire wants to collaborate with Raj on the science portions of a movie script.

Chen

Chen (James Hong) is an elderly waiter at the Golden Dragon in Season 1 until it was revealed to be shut down the following season. He is not to fond of Sheldon after he accused the restaurant of passing off Orange Chicken as Mandarin and refers to Howard as the guy who thinks he can speak Chinese.

Dan

Stephen Root

Dan (Stephen Root) is a senior member of staff at Zangen, who first appears in "The Locomotion Interruption" when he interviews Penny for a pharmaceutical sales representative position as a favor to Bernadette.

Dave Gibbs

Stephen Merchant

Dave Gibbs (Stephen Merchant) is a tall British man who dates Amy after she breaks up with Sheldon. He is recently divorced, his wife having had an affair with a French chef, which he remains bitter over. To Amy's horror, he is an avid fan of Sheldon's work, constantly asking questions about him after learning that he and Amy dated.

Denise

Lauren Lapkus

Denise (Lauren Lapkus) is the assistant manager at Stuart's comic book store introduced in season 11. She is quite a comic book geek. At first, Sheldon does not like the change that her hiring brings to the store but starts to like her because of her comic book knowledge and her ability to figure out his preferences in comic books. By Season 12, she and Stuart enter into a relationship after he takes her as his plus-one to Sheldon and Amy's wedding.

Dr. Eric Gablehauser

Mark Harelik

Dr. Eric Gablehauser (Mark Harelik) is the head of the Physics Department, making him the boss of the guys. Gablehauser later hosts the Physics Bowl,^[21] and gives Raj attention when he is featured in a People article for discovering a planet.

Dr. Gablehauser is well acquainted with Dr. Cooper, Dr. Hofstadter, Dr. Koothrappali and Mr. Wolowitz.

V. M. Koothrappali

Brian George

Doctor V. M. Koothrappali (Brian George) is Raj's father in India. He is a gynecologist and a wealthy man in a 3rd world country. His wife and he communicate with their son via Skype and constantly try to arrange dates for him. They want their son to marry a woman of Indian descent and give them grandchildren.^[8]

Mrs. Koothrappali

Alice Amter

Mrs. Koothrappali (Alice Amter) is Raj's mother in India. Mrs. Koothrappali is especially worried that, despite Raj being old enough to marry, the closest they have to a daughter-in-law is "that little Jewish boy Howard".^[22]

Janine Davis

Regina King

Janine Davis (Regina King) is a human resources representative working at the university. She first appears in season six after Sheldon offends his assistant Alex while trying to deal with her crush on Leonard, leaving Mrs. Davis to handle Alex's sexual harassment complaint against Sheldon. When a tenured position is made available, she is pestered by Leonard, Sheldon, and Raj and wooed by Kripke.

Kurt

Kurt (Brian Patrick Wade) is a tall, intimidating bodybuilder and Penny's ex-boyfriend at the beginning of the series.

Lucy

Kate Micucci

Lucy (Kate Micucci) shows up at a party at the comic-book store on Valentine's Day at an event for people who had no dates. She has social anxiety issues, which allows Raj and her to connect.

Originally, Kate Micucci had been one of the actresses considered for the Amy Farrah Fowler character.

Mike Rostenkowski

Mike Rostenkowski (Casey Sander) is Bernadette's father and a retired police officer. Mike has a rough and rude personality and often bullies others, although he deeply loves his daughter, calling Bernadette his "little girl". Before he is first seen, Bernadette establishes his personality by telling Howard a long list of subjects he cannot discuss with Mike, including Jimmy Carter, foreigners, homosexuals, and even Howard's Jewish identity. Mike oddly enough gets on very well with Sheldon, filling a role in his life vacated by his deceased father, George.

President Siebert

Joshua Malina

President Siebert (Joshua Malina) is the president of the California Institute of Technology. Often mentioned by Sheldon, a recurring gag is that Sheldon often calls or visits Siebert in the middle of the night to get information and usually gets a bitter response. In season eleven, the president returns to support Sheldon and Amy's work in physics.

Priya Koothrappali

Priya Koothrappali (Aarti Mann) is Raj's younger sister. Having graduated at the top of her class at the University of Cambridge (where, like Leonard, her roommate was from Texas), she is one of the lead attorneys at India's biggest car company. Leonard and Priya date for much of season 4 and part of 5 when she moves to Los Angeles, but break up when Priya moves back to India and cheats on Leonard.

Ramona Nowitzki

Riki Lindhome

Ramona Nowitzki, Ph.D. (Riki Lindhome) first appears in the season 2 episode "The Cooper–Nowitzki Theorem", when she is a graduate student at Caltech. She is a huge fan of Sheldon's work and develops a crush on him. She blew him off after Sheldon refused to share credit with her on a theorem he developed. She later reemerged in the Season 10 finale where she kissed him, only for him to further pursue Amy and propose to her.

Ruchi

Ruchi (Swati Kapila) is a new co-worker of Bernadette's at Zangen who first appears in season 11. Ruchi used to live in India, and was in an arranged engagement, until she broke off the engagement and moved to America.

Stephanie Barnett

Sara Rue

Stephanie Barnett, MD (Sara Rue) is a doctor and highly distinguished surgical resident at Fremont Memorial. She did her medical internship at Lawrence Memorial in Galveston, Texas, which is where Sheldon was born.^[23]

Stephanie is picked up in a bar by Howard, but when his plan to let her drive the Mars rover fails, she leaves the Mars Rover lab with Leonard and the two immediately show an attraction to each other.^[24] Howard becomes angry when he finds out, but he soon changes his mind when Stephanie sets Howard up with one of her friends. Sheldon begins meddling in Stephanie and Leonard's new relationship in an effort to strengthen it since in his eyes, Stephanie is the only "tolerable" mate Leonard has had.^[23] After several weeks of dating, Stephanie officially begins living with Leonard in Sheldon and his apartment (although Leonard does not realize this until Sheldon and Penny point it out to him). Stephanie uses sex to manipulate Leonard when he tries to slow down the relationship. After much hesitation, Leonard texts her at the university and tells her to move out because he is uncomfortable with the pace of the relationship, although he apparently leaves to have sex with her. They are not seen breaking up on camera and she is neither seen nor mentioned again.^[25]

Series cocreator Bill Prady said of the relationship: "Stephanie was a chance for Leonard to learn that just because someone loves you, doesn't mean you'll love them back".^[26]

Wil Wheaton

Wil Wheaton, who plays a fictionalized version of himself

Wil Wheaton plays a fictionalized version of himself. Wil played Wesley Crusher in Star Trek: The Next Generation, a character that Sheldon idolized as a child. In 1995, Wheaton made Sheldon's "mortal enemy" list after the former failed to show up at a fan convention as advertised. Sheldon took a 10-hour bus ride to attend the convention, while breaking his own personal rule of using the facilities on board the bus, and was disappointed that Wheaton was a no-show.

During a Mystic Warlords of Ka-ha match at Stuarts comic book store, Sheldon sought revenge against Wheaton by beating him in the game. After revealing that he was at the Dixie Trek convention, Wheaton made a sob story of his grandmother's passing. Sheldon bought the act and threw the game only to find out that Wheaton lied and won the match. Sheldon further vowed vengeance.

Wil Wheaton, LeVar Burton, Leonard Nimoy (in voice-over only), Brent Spiner, William Shatner and George Takei have all appeared on the show, making the Star Trek franchise the most represented franchise on the show in terms of guest appearances.

Marissa Johnson

Marissa Johnson (Lindsey Kraft) is married to Penny's ex-boyfriend, Zack Johnson. They meet with Leonard and Penny and they want Leonard to father their child since Zack is infertile.

Mrs. Wolowitz

Mrs. Debbie^[27] Malvina^[28] Wolowitz (voiced by Carol Ann Susi) is Howard's over-protective, controlling, belittling, but loving and caring stereotypical Jewish mother. The character is not shown on-screen with only two exceptions: firstly in one episode in season six where her body but not her face is briefly shown, and secondly in the Season 5 finale where she can be seen (albeit very small) in the Google Earth satellite picture taken during Howard and Bernadette's wedding. Mrs. Wolowitz's raspy voice is heard usually in scenes at her house, which she shares with Howard. She talks to Howard always by yelling at him from another room, which results in awkward conversations with Howard yelling back at her. In a flashback, when the guys hear her yelling in a masculine voice, they ask Howard if that was his father, to which Howard replies, "If she grows any more hair on her face, yes." Mrs. Wolowitz is also described as being enormously obese and wearing a wig and painted-on eyebrows. She seems to be oblivious to Howard's work as an engineer and treats him as though he is still a child. She is a master at Wheel of Fortune and guessing answers at game shows. She frequently refers to Leonard, Sheldon, and Raj as Howard's "little friends" (as in, "I made some cookies and Hawaiian Punch for you and your little friends!") and often refers to Caltech as a "school". Howard often makes references to his mother as being a "crazy old lady", but deep down he is deeply attached to her and quite happy to be treated like a child. She is also noted to be on hormone replacement therapy, and she wants to "play doctor" with Raj when he asks to sleep over.

Due to the death of her actress, Mrs. Wolowitz character is retired with the fact that she passed away in her sleep while visiting family. Her death hit her son hard, though he is comforted by his friends. It is later revealed the gang were devastated too and paid tribute to her.

Wyatt

Keith Carradine

Wyatt (Keith Carradine) is Penny's father. He comes to visit, and Penny feels compelled to involve Leonard in a ruse to make him think they are still together. When the ruse fails, Wyatt feigns anger at Leonard, but as soon as Penny is out of earshot he begs him to try to win her back. (Wyatt reveals that he favors the successful Leonard over Penny's other "loser" boyfriends.)

Zack Johnson

Brian Thomas Smith

Zack Johnson (Brian Thomas Smith) is a dim-witted but friendly beefcake type whom Penny dates on and off after her second break-up with Leonard. He takes a lively interest in the boys' scientific exploits, but sometimes (and much to Penny's disgust) becomes the butt of their jokes. He did attend college, though it is safe to say that he was a pure jock.

Zack works as the menu designer for restaurants that are owned by or which have hired his father's company to design their menus. Later, Penny and Zach go out to a party, but she is so put off at how stupid he is that she runs back to Leonard for sex, saying he ruined her ability to tolerate stupid men. In season four, Zack runs into Penny, Bernadette, and Amy at a restaurant, and Amy becomes physically attracted to him. Sheldon arranges a meeting between Zack and Amy to satisfy her "urges", but his lack of intellect immediately discourages Amy.

Later, Penny runs into Zack again after he buys her, Bernadette and Amy drinks. He reveals that he has sold his menu company and become rich from the profits, and invites her and Leonard to dinner at his yacht with him and his new wife, Marissa Johnson.

Brenda Sparks

Brenda Sparks (Melissa Peterman) is Billy Sparks's mother who works at the bowling alley frequented by Meemaw. She frequently clashes with Mary due to her religious status. Brenda works at the bowling alley and even starts to have an affair with George Sr.

Thomas

Thomas is the son of Rose and John, brother of Connie Tucker and Kathy, brother-in-law of Charlie Tucker, maternal uncle of Charlene, Carl, Edward Tucker, and Mary Cooper, uncle-in-law of George Cooper Sr., maternal granduncle of Sheldon Cooper, Missy Cooper, and George Cooper Jr., granduncle-in-law of Missy's ex-husband, Missy's second husband, Amy Farrah Fowler, and Mandy Mccalister, and the maternal great-granduncle of Missy's son, Missy's second child, Missy's third child, Missy's fourth child, and the paternal great-granduncle of Leonard Cooper, Constance Cooper, and Sheldon's second child. He was mentioned in the episode Pilot#Young Sheldon (2017) when he attended a pilot with his niece Mary Cooper in 1973.

Rose

Rose is the mother of Connie Tucker, Thomas, and Kathy, mother-in-law of Charlie Tucker, maternal grandmother of Charlene, Carl, Edward Tucker, and Mary Cooper, grandmother-in-law of George Cooper Sr., maternal great-grandmother of Sheldon Cooper, Missy Cooper, and George Cooper Jr., great-grandmother-in-law of Mandy Mccalister, Amy Farrah Fowler, Missy’s ex-husband, and Missy’s second husband, and thus the great-great-grandmother of Leonard Cooper, Sheldon’s second child, Constance Cooper, Missy’s son, Missy’s second child, Missy’s third child, and Missy’s fourth child.

Dale Ballard

Coach Dale Ballard (Craig T. Nelson) is the owner of a sporting goods store and a part-time baseball coach who lets Missy play after he declined but Meemaw intervened. He also ends up dating Meemaw after her breakup with Dr. Sturgis.

Dr. John Sturgis

Dr. John Sturgis (Wallace Shawn) is a physics professor at the university East Texas Tech, which Sheldon attends part-time. Dr. Sturgis is initially a pen pal of Sheldon Cooper before the child prodigy audits his course on quantum chromodynamics. Dr. Sturgis is instantly smitten with Sheldon's grandmother, and they begin dating. He is also rather childish like going to bed at 7:30 and asking Meemaw's permission to do things as if she were his mother. They later split up but remain friends.

Herschel Sparks

Herschel Sparks (Billy Gardell) is a neighbor of the Cooper family and Meemaw, the husband of Brenda Sparks, and the father of Billy and Bobbi Sparks. He runs his own garage, where he offers Georgie a part-time job. Georgie turns out to be a wonderful employee, something that makes George and Herschel happy and stunned. Herschel is often bossed around by his wife, something that he and George bond over since George is often bossed around by Mary as well. In the fourth season, he and Brenda get a divorce.

Paige

Paige Swanson (Mckenna Grace) is a 10-year-old child prodigy whom Sheldon meets in one of his classes with Dr. Sturgis. As his rival, she teases him about differentiating under the integral sign. She has higher social skills than Sheldon and is capable of being friends with nonintellectuals. She reacts negatively to her parents' divorce. Dr. Sturgis advises Sheldon to listen to her problems. He does, which results in Sheldon learning to listen. He attempts to console her by making her a hot beverage.

Robin

Robin (Mary Grill) is a police officer and Pastor Jeff's second wife. Pastor Jeff meets Robin in "A Broken Heart and a Crock Monster" when she stops him for passing a red light. She takes an interest in him and because of that, refuses to give him a ticket.

Tam Nguyen

Tam Nguyen (Ryan Phuong – YS / Robert Wu – TBBT) is a Vietnamese-American Catholic boy who, in Young Sheldon, is Sheldon's classmate and only friend. His father Le Nguyen, and mother Trang Nguyen, run a store called "Medford Mart". He has a troubled relationship with his parents, having been treated badly by both of them after moving to the United States. He has two sisters, Mai and Kim-Ly, with whom he has a good relationship. Tam laments his difficult childhood in war-torn Vietnam, which included his father being incarcerated in a re-education camp to "become a good communist", and his encounter with the Ku Klux Klan when he first arrived in America.

Veronica Duncan

Veronica Duncan (Isabel May) is Georgie Cooper's most significant love interest on Young Sheldon. Initially introduced as a popular and "easy" girl, Veronica becomes a devout Christian after going through a "Hell house" one Halloween. Georgie is obsessed with her and frequently takes every advantage he can to be close to her or impress her in any way.

The Big Bang Theory

• Kareem Abdul-Jabbar: "The D&D Vortex"
• Buzz Aldrin: "The Holographic Excitation"
• Frances Arnold: "The Laureate Accumulation" (12th season)
• LeVar Burton: "The Toast Derivation", "The Habitation Configuration", "The Champagne Reflection"
• Ellen DeGeneres: "The Geology Elevation", "The Laureate Accumulation"
• Nathan Fillion: "The Comic Book Store Regeneration"
• Carrie Fisher: "The Convention Conundrum"
• Ira Flatow: "The Vengeance Formulation" (voice only), "The Discovery Dissipation", "The Retraction Reaction"
• Neil Gaiman: "The Comet Polarization"
• Bill Gates: "The Gates Excitation"
• Sarah Michelle Gellar: "The Stockholm Syndrome"
• Summer Glau: "The Terminator Decoupling"
• Brian Greene: "The Herb Garden Germination"
• Mark Hamill: "The Bow Tie Asymmetry"
• Stephen Hawking: "The Hawking Excitation", "The Extract Obliteration", "The Relationship Diremption", "The Troll Manifestation", "The Celebration Experimentation", "The Geology Elevation", "The Proposal Proposal"
• James Earl Jones: "The Convention Conundrum"
• Stan Lee: "The Excelsior Acquisition"
• Howie Mandel: "The Re-Entry Minimization"
• Joe Manganiello: "The D&D Vortex"
• Mike Massimino: "The Friendship Contraction", "The Countdown Reflection", "The Decoupling Fluctuation", "The Re-Entry Minimization", "The Table Polarization", "The First Pitch Insufficiency"
• Elon Musk: "The Platonic Permutation"
• Adam Nimoy: "The Spock Resonance"
• Bill Nye: "The Proton Displacement", "The Conjugal Configuration"
• Samantha Potter: "The Panty Piñata Polarization"^{[lower-alpha 6]}
• Katee Sackhoff: "The Vengeance Formulation", "The Hot Troll Deviation" (both appearances are in a daydream scene)
• William Shatner: "The D&D Vortex"
• Charlie Sheen: "The Griffin Equivalency"
• Kevin Smith: "The Fortification Implementation", "The D&D Vortex"
• George Smoot: "The Terminator Decoupling"
• Brent Spiner: "The Russian Rocket Reaction"
• George Takei: "The Hot Troll Deviation" (appears in a daydream scene)
• Kip Thorne: "The Laureate Accumulation"
• Analeigh Tipton: "The Panty Piñata Polarization"^{[lower-alpha 7]}
• Neil deGrasse Tyson: "The Apology Insufficiency", "The Conjugal Configuration"
• Adam West:^[29] "The Celebration Experimentation"
• Steve Wozniak: "The Cruciferous Vegetable Amplification"

Young Sheldon

• David Hasselhoff: "Cowboy Aerobics and 473 Grease-Free Bolts"
• Stephen Hawking: "The Grand Chancellor and a Den of Sin" (voice only)
• Cyndi Lauper: "A Baby Tooth and the Egyptian God of Knowledge" (voice only, saying the words of the title of her song, "girls just want to have fun")
• Elon Musk: "A Patch, a Modem, and a Zantac®" in a flashforward scene set 27 years into the future.