1993–2007

Sonia and her family move to Albert Square. She is the third of Carol Jackson's (Lindsey Coulson) four children and the result of her mother's relationship with Terry Cant, whom Sonia never knew and who was violent towards Carol. During her childhood, she becomes close friends with Clare Tyler (Gemma Bissix) and the two are inseparable until Clare falls in with the wrong crowd at school and starts bullying Sonia but they reconcile before Clare leaves in 1998. Sonia has sex with Martin Fowler (James Alexandrou) but starts dating Jamie Mitchell (Jack Ryder). Not knowing she is pregnant, Sonia goes into labour and gives birth to Martin's daughter. The baby, who is named Chloe, is subsequently given up for adoption and renamed Rebecca, despite protests from Martin's mother Pauline Fowler (Wendy Richard). However, Sonia becomes obsessed with her daughter and abducts her, locking herself in her house with the baby. Despite pleas from Rebecca's adoptive parents, it is Sonia's step-grandmother, Dot Cotton (June Brown), who persuades Sonia to return the baby.

Sonia and Jamie get engaged, but due to Jamie's fling with Zoe Slater (Michelle Ryan) and constant rowing, the relationship ends. Sonia dates Gus Smith (Mohammed George), who plans to propose, but he is devastated when Sonia reconciles with Jamie after he is brutally assaulted by his godfather Phil (Steve McFadden). Sonia helps him, nursing him through his injuries and they decide to marry but their happiness is short-lived as Martin accidentally hits Jamie with his vehicle. Sonia keeps a bedside vigil and is with Jamie when he dies.

Sonia eventually forgives Martin and as they grow closer, they fall in love and eventually elope. Sonia begins nursing training and she and Martin live with Pauline but struggle to cope with her interference. Martin gets unwittingly involved with a stalker, Sarah Cairns (Alison Pargeter), who tries to ruin his marriage. This culminates in Sarah holding Sonia and Martin hostage and stabbing Martin, until Sonia knocks her unconscious. Sonia and Martin face further upset when they discover that their daughter's adoptive parents have died in a car crash. They visit Rebecca's (Jade Sharif) guardian, Margaret Wilson (Janet Amsden), hiding their true identity but when Margaret realises who they are, she asks them to leave. Pauline also interferes and visits Rebecca with Martin. Sonia is furious when she learns this and animosity in the Fowlers' home puts a strain on Martin and Sonia's relationship so she spends her time with a colleague, Naomi Julien (Petra Letang); feelings develop and they begin an affair. Martin is heartbroken when his marriage ends and the situation is complicated further by Rebecca, who, on Margaret's request, is spending more time at the Fowlers'. Margaret decides to make the Fowlers' Rebecca's legal guardians but does not include Sonia when she finds out about her lesbian fling. Margaret dies and Martin takes custody of Rebecca and Pauline tries to stop Sonia having any contact with Rebecca. This makes Sonia distraught and preoccupied with Rebecca. Feeling neglected, Naomi ends their relationship.

Sonia becomes depressed and begins to neglect her work and drink alcohol, until Gus helps her straighten her life out. Sonia and Gus begin a second romance, which ends due to Sonia's lingering feelings for Martin. Sonia and Martin receive their divorce papers. However, both regret getting divorced and they reconcile. Pauline eventually finds out and tells Martin that she is dying from a brain tumour to stop him moving in with Sonia. Just as Pauline intends, Martin ends his relationship with Sonia in order to care for her, but when the truth finally comes out, a furious Martin moves out anyway. Pauline responds with ire, threatening to cut her son from her life. Sonia tries to build bridges with Pauline but an argument ends with Sonia slapping Pauline. When Pauline is found dead later that day, Sonia fears that she caused her death. She discovers that Rebecca witnessed the slap and tries to silence her daughter, but Rebecca tells Dot at Pauline's funeral. Appalled, Martin refuses Sonia access to Rebecca. Fearing imprisonment, Sonia abducts Rebecca and goes on the run. Martin is frantic, but days later Sonia returns and she is arrested on a murder charge; however, that same night Dot uncovers the identity of Pauline's real murderer — Pauline's husband Joe Macer (Ray Brooks), who had argued with Pauline on Christmas Day and struck her over the head, causing her death. Sonia is released and decides to leave Walford for Manchester. After making peace with Martin, she takes a taxi and is about to leave Walford when she sees Martin and Rebecca obstructing the road; Martin has decided that he wants to go with Sonia. The Fowlers reunite and leave as a family. They remarry and spend time touring the USA before settling in Manchester.

2010–2011

Bianca invites Sonia and Martin to her wedding to Ricky Butcher (Sid Owen) and Sonia attends alone. She rejects a phone call from Martin and gets very drunk. The following day, Sonia walks in on Dot slapping her granddaughter Dotty Cotton (Molly Conlin) across the legs, leading to Dot's arrest, though she is released without charge. Bianca notices Sonia has continuously avoided talking about Martin and Rebecca. Sonia visits Jamie's grave and later returns to The Queen Victoria pub, and drunkenly berates Phil, Jamie's godfather, about his treatment of Jamie. Phil says he considers Jamie to be one of his own children and tells Sonia to go upstairs to sober up. She misunderstands him and gets into his bed, naked. As Phil sees Sonia in bed, Shirley Carter (Linda Henry) walks in and slaps Phil across the face. Sonia confides in Carol about her marriage problems and Carol advises her to try to stay together for Rebecca's sake. After Ricky and Bianca's wedding, Sonia decides she must make a fresh start with Martin and leaves again. Sonia returns in January 2011, following Bianca's disappearance following her attack on Connor Stanley (Arinze Kene), who had also dating Whitney. She reveals to Ricky and Carol that Bianca has been hiding out with her at a B&B and is not in a good way. Ricky suggests getting Bianca out of the country but Sonia tells him she that Bianca has decided to give herself up. During a conversation with Carol, Sonia reveals she and Martin are no longer together and she and Rebecca are back living in London while Martin remains in Manchester. Carol is surprised but Sonia tells her the split occurred around the time of Billie's death and did not want to burden her with her problems. After Bianca hands herself in, Sonia leaves again.

2014–present

In January 2014, Carol informs Sonia, who is having marital problems with Martin again, about her breast cancer diagnosis. Carol has the BRCA gene and Sonia finds out she has inherited it. Sonia kisses Tina Carter (Luisa Bradshaw-White), which Tina's girlfriend, Fiona "Tosh" Mackintosh (Rebecca Scroggs), finds out about, so Tina end her friendship with Sonia to protect her relationship. Sonia, who has started a weight loss class, arranges to have a gastric band in Bulgaria, telling friends and family she is on a course. When she returns, she tries to confess to her class but collapses before she can do so. Sonia and Martin (now played by James Bye) separate and Sonia starts to feel isolated and depressed. Sonia is supported by Tina but Sonia backs away when Tina goes to kiss her as Sonia is unsure of her feelings. Martin moves back to Walford and Rebecca (now played by Jasmine Armfield) moves in with him, as she resents Sonia for leaving Martin. Martin believes he can repair his marriage, but Tina and Sonia share a kiss, which Martin sees. He tells everyone in The Queen Vic about Sonia and Tina, and Sonia ends their relationship. Martin sends Rebecca, who now wants to be called Bex, to live with Sonia so she can spend more time with her mother, whilst Sonia also repairs her relationship with Tina. Martin begins dating Stacey Branning (Lacey Turner), and Sonia also decides to move on by embarking on a full relationship with Tina. Sonia and Martin's divorce is later finalised and he marries Stacey, with Sonia giving her blessing. After Tina drunkenly humiliates Sonia at an awards ceremony and Sonia rejects her, Tina has sex with Sophie Dodd (Poppy Rush) and regrets it, but before she can tell Sonia, Sonia reveals she has found a lump in her breast. Sonia forgives Tina for the humiliation, but Sophie tells Sonia that Tina cheated and their relationship ends, but they soon reunite. After having her breast lump checked, Sonia is given the all-clear. Sonia supports Tina by looking after her ailing mother Sylvie Carter (Linda Marlowe) and ends up becoming her full-time carer.

In September 2016, Sonia receives a lucrative job offer at a private hospital in Kettering and, having realised she no longer loves Tina, ends their relationship. Bex is initially furious and thinks that Sonia is abandoning her, but later calms down and she says an emotional farewell. During Sonia's absence, Bex is bullied by Madison Drake (Seraphina Beh), Alexandra D'Costa (Sydney Craven) and her former friend Louise Mitchell (Tilly Keeper). Bex breaks down and calls Sonia, prompting her to return, and she accuses Martin and Stacey of not looking after Bex. Sonia decides to take Bex with her along to Kettering and she meets with Bex's headteacher at Walford High in order to report Madison and Alexandra's actions, but Sonia decides not to mention Louise's involvement at Bex's request. When she realises that Bex is happy staying with Martin and Stacey, Sonia returns to Kettering. Later Sonia returns to Walford and lets herself into Dot's house, finding Dot on the floor after tripping. Sonia and Robbie, who has also returned, move in with Dot to help care for her. It is revealed that Sonia is hiding a secret from her time in Kettering as she no longer choses to return there. Although Sonia wants Bex to move in with them, Bex chooses to stay with Martin and Stacey. Mr Waters (Andrew Bone), the son of one of Sonia's former patients in Kettering arrives in Walford and accuses Sonia of fleecing his late mother out of her money. Sonia reveals that she had been nursing her and inherited everything in her will, but nobody believes her and Bex assumes that she has run away from Kettering. Bex's teacher Gethin Pryce (Cerith Flinn) defends Sonia and orders Mr Waters to leave. Sonia starts dating Gethin but is unaware that Bex is infatuated with him. Bex and Gethin kiss and after Bex attempts to pursue him, he eventually confesses the truth to Sonia. Gethin subsequently resigns his job at the school and leaves Walford.

In December 2017, Sonia discovers Kim Fox (Tameka Empson) having a miscarriage and supports her. Martin and Stacey's marriage is fractured when Stacey sleeps with Sonia's uncle Max Branning (Jake Wood). Stacey kicks Martin out and Sonia agrees for him to move into No. 25 with her and Bex. Sonia discovers that she may still be in love Martin and agrees to go on a date with him. However, Martin still has feelings for Stacey, which upsets Sonia. When Bex decides that she doesn't want to go to university, Sonia pressures Bex into applying to Oxbridge University, much to Bex's annoyance. Bex tells Sonia that she is feeling upset since her boyfriend Shakil Kazemi (Shaheen Jafargholi) was killed and that she isn't ready for university, and Sonia supports Bex's decision. Sonia and Dot care for Harold Legg (Leonard Fenton) when he becomes ill and he moves into No.25. A swastika is graffitied on Dot's door which causes Dr Legg to fall into a panic attack. Sonia blames Dot's new lodger Stuart Highway (Ricky Champ) for the attack and threatens to report him to the police if he comes anywhere near No. 25. However Stuart is innocent and the perpetrators behind the attack are young youths. Sonia's niece Tiffany Butcher (Maisie Smith) is involved in a drug ring gang and Sonia discovers this when Tiffany's friend Bernadette Taylor (Clair Norris) informs her. Sonia and the Brannings try to help Tiffany and promise to pay off the drug dealers that Tiffany and her friend Evie Steele (Sophia Capasso) owe money to, to stop Tiffany from running away. Tiffany discovers that Sonia is lying and attempts to leave, but Sonia stops her from leaving causing Tiffany to slap her. She locks Tiffany in Dot's from and finally releases her when she agrees to speak up about the drug ring.

When Bex takes an overdose, Sonia finds her and is dismayed when she realises that it was a suicide attempt. She blames herself, and when Martin returns to Walford, Sonia is angry with him for being absent. She is forced to give him a false alibi when police question him about stolen vehicles and she convinces him to tell the truth. She forgives him but Martin's behaviour continues and he is injured during an attack so Sonia throws him out. She forgives him again and realises that she still has feelings for him. When Martin and Charlie "Tubbs" Savage (Tayla Kovacevic-Ebong) knock down George Watson (Jack Bennett), Sonia visits George in hospital to question him about the hit-and-run. Upon realising Sonia and Martin are in a relationship, George blackmails Sonia and threatens to report Martin to the police unless she gives him £10,000. Sonia steals the money from Dot's bank account.^[7] Sonia is guilty but Martin decides to take the blame. Dot throws him out and leaves Walford to live with her grandson Charlie Cotton (Declan Bennett) and his wife Liz (Michelle Connolly) in Ireland.

In June 2020, Sonia decides she wants to get in touch with her birth father, Terry Cant. However, the subsequent COVID-19 lockdown means that she is busy at the hospital, which leaves Sonia stressed and exhausted. When Ian Beale (Adam Woodyatt) makes light of the pandemic, Sonia snaps at him and decides to visit her family abroad. During her 36th birthday, Sonia meets Tom "Rocky" Cotton (Brian Conley) who is impersonating her father, Terry. He tells her that he loved Carol but Sonia is unwilling to build bridges due to Terry’s abusive treatment of Carol when they were together and she sends him away. Rocky returns and decides to stay to establish a relationship with Sonia, and they eventually bond. Unknown to Sonia, Rocky's real name is Thomas Cotton, and he is working with his niece, Dotty Cotton (Milly Zero), to con Sonia who is in control of Dotty's inheritance since Sonia has power of attorney. Rocky later reveals the truth and Sonia rejects him, although she forms a friendship with Rocky’s partner Kathy Beale (Gillian Taylforth). Sonia allows Janine Butcher (Charlie Brooks) to live with her and supports her when Social Services investigate Janine as a mother to her daughter, Scarlett (Tabitha Byron). Janine is suspicious that her boyfriend Mick Carter (Danny Dyer) is cheating with his ex-wife Linda (Kellie Bright), and Sonia investigates Mick's behavior and initially assures Janine that Mick loves her until she sees him embrace Linda. Janine lies to Mick that she is having complications with her pregnancy after a fall at the Tube Station in order to push him away from Linda, but Sonia encourages Janine to be honest with Mick, causing Janine to threaten her. Out of concern for Janine, Sonia calls her former brother-in-law (and Janine’s brother) Ricky back to Walford.

In December 2022, Sonia receives a phone call from Charlie with the news that Dot has died. As Charlie is ill, Sonia arranges Dot's funeral with the support of local residents and she contacts some of Dot's old friends who return to pay their respects at the funeral. She becomes close to Dot's relative, Reiss Colwell (Jonny Freeman) and they sleep together after the wake. Sonia discovers that she has inherited the majority of Dot’s estate, including her house, much to her disbelief and Dotty’s anger, which results in Dotty leaving Walford. Sonia spends Christmas Day with Janine, Ricky, and the Carters at The Queen Vic, and is shocked when Scarlett reveals that Janine had framed Linda for a car crash, and Jada Lennox (Kelsey Calladine-Smith) also reveals that Janine had paid her to plant wine bottles in Linda's bag and call social services to take her children. Janine decides to flee when Mick calls the police and she abandons Scarlett with Sonia and Ricky. After Mick is presumed dead from drowning in the sea during a car chase with Janine and Linda, Janine returns and tries to manipulate Scarlett into retracting her statement but Ricky reports her to the police leading to her arrest. He soon returns to Germany with Scarlett, leaving Sonia alone. She decides to invite Reiss to live with her and they begin a relationship. Sonia discovers that Rocky is still married to his first wife, Jo Cotton (Vicki Michelle), and she forces Rocky to tell Kathy. However, Sonia is unaware that Reiss is also married to his wife Debbie (Jenny Meier) and he hides this from Sonia. Ironically Rocky overhears Reiss talking about his wife and also forces Reiss to admit this to Sonia. Sonia proceeds to evict Reiss and he reveals that his wife is in hospital after having a stroke that has left her unresponsive. Sonia follows Reiss to visit Debbie in hospital but Sonia cannot forgive Reiss for lying to her; she realises that she loves him and they reconcile.

In August 2023, Sonia worries that she is pregnant and is convinced by Bianca's stepdaughter Whitney Dean (Shona McGarty) to take a pregnancy test. Sonia is reluctant when Reiss states that he doesn't want children, but her test is negative. Reiss discovers the test and believing that he cannot be the father, he assumes that Sonia has been having an affair with Whitney's boyfriend Zack Hudson (James Farrar). Reiss reveals that he suffers from low sperm count and cannot have children naturally, and tried to have IVF with Debbie before her stroke. He then asks Sonia to have his child via IVF and she agrees.

Departure (2007)

In April 2006, it was announced that Cassidy would take a break from EastEnders, saying "it's time to try new things. But it doesn't mean Sonia won't be back".^[10] A BBC spokesperson added, "Natalie is a valued member of the EastEnders cast and we wish her all the very best during her break from the show".^[10] Sonia departed on screen in February 2007, along with her former husband Martin, as his actor, James Alexandrou, also left the serial. Their departing storyline was the culmination of the death of Pauline Fowler, following actress Wendy Richard's high-profile resignation from the serial in 2006. In the storyline, Pauline dies in suspicious circumstances and Sonia is blamed for killing her, although Sonia is eventually vindicated when Pauline's husband Joe Macer (Ray Brooks) admits to manslaughter. Sonia and Martin and their daughter Rebecca leave Walford as a united family for the United States. In 2010, Cassidy said one of the reasons she decided to leave the show was because of Sonia's affair with Naomi Julien (Petra Letang), saying it was wrong for the character.^[11]

Despite Cassidy's departure being specified as a "break", Cassidy commented in 2008, "I feel that door's closed now and I need to go and do other stuff [...] Maybe in nine years I might go back."^[12] More recently, in 2009, press reports suggested that the actress was keen to reprise the role. Patsy Palmer, who returned to EastEnders as Sonia's sister Bianca, publicly called for the actress's return: "I would love her to come back. I mean, she is family! I've told bosses that it's a good time, when you think about it, as the Jacksons are back now as well."^[13]

Guest stints (2010–2011)

On 25 October 2009 it was announced that Cassidy would return to the soap to reprise her role as Sonia, along with Coulson, Gaffney and Anderson. The characters would reunite with Bianca who returned in 2008. Cassidy is quoted as saying "To be invited back to EastEnders is such an honour and I am very, very excited that Sonia is coming back. For me, to be reunited with the original Jacksons is a dream come true and I think it will be a fantastic storyline."^[3] A source told entertainment website Digital Spy: "Everything's being kept hush-hush about the Jacksons' return at the moment, but there are certainly a load of questions to be answered. Will the frosty relationship between Carol and Bianca thaw? Are Sonia and Martin still together? And how will Robbie react when he learns of his beloved Wellard's death? Everyone's so pleased to have them back."^[3] The family returned for Bianca's second wedding to Ricky Butcher as part of the show's 25th anniversary. Executive producer Diederick Santer explained that he wanted great stories to get people talking, saying "The wedding is the perfect opportunity for us to bring back the much-loved Jackson characters – Carol, Sonia, Robbie and Billie."^[14] In December 2010, it was announced Cassidy would return to EastEnders,^[15] as Sonia in order to facilitate a temporary storyline departure for co-star Palmer, who was due to take maternity leave.^[16] Sonia's one-off return was broadcast on 21 January 2011.^[17]

Reintroduction (2014)

Sonia returned permanently on 14 January 2014.^[18][19]

In February 2016, Cassidy announced she was expecting her second child, meaning she would be taking maternity leave. Cassidy confirmed the baby is due in "summer" 2016.^[5] It was expected that Cassidy would leave in spring 2016, with Sonia departing on-screen in summer 2016. Sonia departed on-screen on 20 September 2016.^[5] After returning for a short stint from 14 April 2017 to 18 April 2017, Cassidy returned to the role of Sonia permanently, with the return being broadcast on 27 June 2017.^[20]