Main

• Jung Kyung-ho as Jung Shi-hyun ("The Doctor's Son") - Grew up in a red light district / brothel until his mom died from drug-overdose. Then ran away from an orphanage (to Jin-sook) and eventually became a drug courier for "Uncle" Safari. Shi-hyun is nicknamed "The Doctor's Son" and became the boss of a mid-sized gang. Highly ambitious. He is plotting to take over the position of "Scale", his drug supplier.
  • Sung Yoo-bin as Shi-hyun (young)
• Lee Jae-yoon as Ji Hyung-min^[3] - Hyung-min graduated from the police academy with top scores and gets promoted at a young age. Was studying law to join the Prosecutor's office, but returned to the police force to avenge an undercover officer's death. Now the section chief of a special investigation unit, his goal is to take down "Scale", which leads him to Shi-hyun.
• Nam Gyu-ri as Yoon Soo-min^[4] - Soo-min grew up in an orphanage with Kyung-mi, and they treat each other like real sisters. When Kyung-mi is killed in the line of duty, Soo-min goes undercover to catch her friend's killer and finish what Kyung-mi started. However, she ends up falling for "The Doctor's Son", the drug lord Kyung-mi was investigating until her untimely death.
• Son Chang-min as Min Hong-ki, Director of the police / special investigative unit.
• Kim Yoo-mi as Lee Jin-sook, President / CEO for various adult entertainment establishments. Worked the streets at a younger age. Close friend to Shi-hyun's mom. Looked out for Shi-hyun as he was growing up. Along with Soo, they both support and follow Shi-hyun's direction / quest to take over the drug distribution channel.
• Choi Moo-sung as Moon Deok-bae ("Safari") - Knew Shi-hyun as a child. Paths did not cross again until he was sent down to Busan to pursue "Doctor's Son". Taught / gave Shi-hyun a switchblade knife when he was younger (this became Shi-hyun's weapon of choice).
• Yoon Hyun-min as Kim Hyun-soo - Met Shi-hyun in prison and both worked under "Halibut". At times, hot headed but loyal to a fault.

Supporting

• Go Na-eun as Lee Kyung-mi, an undercover agent. She is Hyung-min's girlfriend and Soo-min's older sister. She grew up with Shi-hyun at the orphanage and is hinted that she is Shi-hyun's love interest, she later on died after being shot in the head by a sniper during an undercover mission to catch "Doctor's Son" who happened to be Shi-hyun.
• Park Soo-young as chief of detective squad
• Kil Yong-woo as Ji Man-hee, Attorney general and Hyung-min's father
• Kim Byeong-ok as Jeoul ("Scale"), Head of the narcotics ring (drugs, loan sharking, adult entertainment establishments). Leader behind the scenes for "Halibut" and "Meth Kim".
• Kim Jung-hak as Ahn Kyung-chan, prosecutor
• Song Min-ji as Cha Hyo-joo, reporter
• Jung Soo-young as Oh Jung-yeon, prosecutor
• Kim Jong-goo as Jo ("Busan") aka "Chairman"

Leader of Busan gang.

• Jung Ji-soon as Jo Ha-neul

Son of "Busan".

• Park Young-ji as Cha

Lawmaker and Hyo-joo's father.

• Jung Moon-sung as Shi-hyun's subordinate
• Kim Min-sang as Kim Bbong ("Meth")
• Kim Hyo-sun as Eun-soo
• Lee Moo-saeng as Detective Kim (Hyunsoo's Sources)
• Shim Min as Joo-young

Soo-min's friend since kindergarten.

• Kim Jae-hwa as Park Eun-ae
• Choi Kwon as Detective
• Lee Kyu-Bok as Park Kyung ho
• Kim Soo-Hwan as Prison Officer Park
• Kim Myung Heon as Sniper