Seo Jin-woo has a special condition called hyperthymesia which allows him to remember almost every day in perfect detail. His father Seo Jae-hyuk, on the other hand, has developed Alzheimer's disease, making him lose his memories. When Seo Jae-hyuk is wrongfully convicted of murder, Jin-woo vows to prove his father's innocence. Four years later, Seo Jin-woo becomes a lawyer to put real criminals behind bars, but bribery, corruption, and betrayal will not allow him an easy road to justice.
As a child, he goes to live with his father after his mother and brother die in a car accident. Using his superior memory, he is able to become the youngest lawyer in South Korea after dropping out of high school to prove his father's innocence and prevent his execution. Four years after meeting Lee In-ah, he becomes her boyfriend. He later suffers from Alzheimer's and constantly experiences sudden memory loss, which worsens throughout the series. This further motivates him to bring Nam Gyu-man and the Il-ho company to justice while simultaneously proving his father's innocence.
Apart from Park Dong-ho, she is the only one to believe that Seo Jin-woo's father is innocent, and she unconditionally supports Jin-woo during tough times. While in university, she helps her parents deliver Papa John's Pizza. Four years later, she becomes a prosecutor in order to fight for Jin-woo's father. She believes in honesty and truth but is hindered by her superiors, who accept bribes from Il-Ho Group. She later prosecutes Gyu-man and is able to successfully prove him guilty of murder, among other crimes, for which he is sentenced to death. She also eventually becomes Jin-woo's girlfriend.
A lawyer with a 100% success rate, infamous for defending the rich and powerful regardless of their guilt or innocence. He is known to speak Korean with a very thick Gyeongsang accent. When he first meets Jin-woo, he is reluctant to take on his father's case due to the high likelihood of losing. But since Jin-woo reminds him of his younger self, he softens his heart and takes on the case. However, he eventually tanks the case under pressure from the Il-Ho Group. He then signs under the Il-Ho legal team, but still supports Jin-woo deep in his heart. It is also revealed that the car accident his father caused many years ago is the same one that killed Jin-woo's mother and brother. Jin-woo eventually lets go of his hatred towards Dong-ho and forgives him, and they team up to expose Nam Gyu-man's crimes and the corruption of the Il-ho Group.
A twisted and wealthy antagonist, Nam Gyu-man is responsible for the crime that Seo Jin-woo's father is wrongly accused of. He is ruthless, violent, and brutal, to the extent that his company maintains a separate legal team to handle the fallout of his behavior. He also has a terrible fear of his father's wrath. Nam Gyu-man goes on to commit more heinous and treacherous acts throughout the series, demonstrating a lack of remorse, disregarding right and wrong, and having no regard for human life. Eventually, Nam Gyu-man is arrested and brought to trial for the murder of Oh Jung-ah among his many other crimes. He is found guilty, convicted, and sentenced to death. In the series finale, he commits suicide by hanging while in solitary confinement on death row.
Living in an upper-class bubble, Yeo-kyung is Nam Gyu-man's sister and is adored by her father. She is also Lee In-ah's classmate and colleague, and is quickly attracted to Seo Jin-woo because of his amazing skills. Unlike her father and brother, she is good-natured and kind-hearted, leading her to express her disappointment in them for their heinous deeds. Near the end of the series, she informs In-ah that her brother is trying to flee to China using a fake passport and identity, which allows In-ah and Jin-woo to stop him and bring him to trial for the murder of Oh Jung-ah and all his other crimes. Yeo-kyung eventually packs up her belongings and leaves her father's home for good in the series finale.
Jin-woo's father. He is wrongly accused of crimes committed by Nam Gyu-man and is subsequently sentenced to death. He suffers from Alzheimer's and is therefore unable to prove himself innocent, and he later dies of cancer. Finally, after two retrials, Seo Jae-hyuk is posthumously acquitted of all criminal charges.
A corrupt chairman of Il-Ho company, and Gyu-man and Yeo-kyung's father. He will stop at nothing to protect his company. He is eventually arrested for corruption in the finale of the series.
Il-Ho company's section chief and Nam Gyu-man's former right-hand man. He was obedient and loyal to Gyu-man and often serves as his punching bag, but does not agree with his extreme methods. Soo-bum eventually allies himself with Jin-woo and plays a crucial role in exposing Gyu-man as the real murderer of Oh Jung-ah.
A corrupt prosecutor who is excellent at fabricating stories during case trials. He handles Seo Jae-hyuk's trial and subsequently moves up the ranks to become Lee In-ah's section chief. He misuses his power to cover up the crimes of Il-Ho company, and abets Il-ho company and Nam Gyu-man in committing further crimes throughout the series. He is also Park Dong-ho's rival. Near the end of the series, he assists the protagonists in bringing Nam Il-ho to justice, providing evidence to expose the corruptions of Il-ho company. Nevertheless, in the series finale he is arrested for corruption and fabricating evidence in Seo Jae-hyuk's trial.
A prosecutor and Lee In-ah's boss. He often reprimands In-ah for not working within the role of a prosecutor, but he nevertheless understands and supports her.
The son of an employee in a small subsidiary of Il-ho company. He hires Jin-woo as his father's lawyer when he is falsely accused of a crime by Il-ho company.
Kim Hyung-beom as Attorney Song Jae-ik
Seo Jae-hyuk's original defence lawyer, later replaced by Park Dong-ho. Since the day Seo Jae-hyuk is sentenced to death, he feels guilty for not helping Jin-woo prove his father's innocence. Four years later, he joins Jin-woo's law firm after Jin-woo becomes a lawyer. He also has a persistent stutter, which he eventually overcomes.
A woman who once worked at Jin-woo's law firm, and whom Jin-woo helped to escape from an abusive marriage. Throughout the series, Bo-mi acts as a mother figure for Jin-woo, and is generally supportive of him.
Jin-woo's neighbour. She is invited to sing at a party hosted by Nam Gyu-man, where she is latter assaulted. Nam Gyu-man murders her after she injures his face with broken glass.
Jung-ah's father. He hates Jae-hyuk for killing his daughter and only child, and even threatens Jae-hyuk with a knife in court, demanding he be put to death. Later he accidentally finds evidence suggesting that Seo Jae-hyuk is not the true culprit, and subsequently informs his attorney, Hong Moo-suk, not knowing that Moo-suk is secretly in the side of Il-Ho company. Because of this he is found dead, presumably by suicide over the grief of his daughter's death. Four years later, Lee In-Ah discovers a letter that Sung-taek left at Oh Jung-ah's memorial. The handwriting in this letter and in his supposed suicide note are not a match, indicating that he was murdered, with Il-ho company playing a part in it.
Nam Myung-ryul as Kang Man-soo
He was the Vice President of Il-Ho company, who was married with one daughter. Kang was accused of sexually assaulting a woman named Kim Han-na. He was innocent of the crime, and the accusation was a scheme by Chairman Nam to get rid of him. Jin-woo took up Kang's case and he managed to successfully prove Kang's innocence. Kang later provided him evidence of Il-ho Company's corruption crimes.
She was a participant of Chairman Nam's scheme to get rid of the Vice President of Il-Ho company. She framed Vice President Kang Man-soo for sexual assault to get money for her mother's operation. Eventually, she admitted in court under Jin-woo's cross-examination that she indeed framed Kang for sexually assaulting her (there was another culprit).
A psychiatrist who was bribed by Il-ho company to fabricate evidence regarding Seo Jae-hyuk's psychiatric condition in his trial.
Kim Young-woong as Kwak Han-soo
A corrupt police officer who accepted bribes from Il-ho company and was an ally to Nam Gyu-man. He was the one arresting Seo Jae-hyuk and forcing him to sign the confession by threatening him the life of his son. Kwak was eventually arrested and imprisoned for the offences he committed, and he later redeemed himself by helping Jin-woo to expose Nam Gyu-man as the one who murdered Oh Jung-ah and becoming a witness in Jae-hyuk's second retrial to testify. He was shown to have found religion in prison and reformed in the finale of the series during his final encounter with Gyu-man (who was then detained on death row before his suicide).
*The broadcast of Episodes 7 and 8 scheduled on 30 and 31 December 2015, respectively, was postponed due to the annual SBS Entertainment Awards and SBS Drama Awards.