Mitsuko Sôma is a student of Shiroiwa Junior High School’s ninth-grade Class B, designated Girl 11 in the Battle Royale Program. She is one of the most dangerous participants, second only to Kazuo Kiriyama in the number of classmates she kills. On the surface, she is strikingly beautiful, with an angelic face and a shapely figure that she consciously uses as a weapon. Beneath that exterior lies a deeply traumatized and hollow individual, shaped by a childhood of extreme abuse and betrayal.

Her backstory varies across adaptations but consistently involves severe sexual and physical abuse. In the novel, when she was nine, her mother accepted money from three men to rape her while they filmed the act. When she confided in a teacher, he also raped her, and a friend who witnessed it spread rumors rather than help. After she accidentally killed her mother while resisting another attempted sale, she was sent to live with relatives, where the father of the household began molesting her. In the manga, her biological father, who genuinely loved her, was forced to flee the country for political reasons, giving her a ring before leaving. She then lived with her mother and a violent stepfather who beat and raped both of them. Eventually, Mitsuko hired a yakuza to kill her parents, then betrayed the killer to the police. In the film, a flashback shows her as a young child returning home to find her mother drunk and a man who had paid for the right to rape her; she pushed him down the stairs to his death. In all versions, these experiences shattered her ability to trust anyone and twisted her understanding of intimacy, leading her to believe that sex is the answer to all problems and that she must take from others before they can take from her.

In school, Mitsuko leads a small gang that includes Hirono Shimizu and Yoshimi Yahagi. They are involved in prostitution, drug use, theft, and extortion. Her classmates both fear and despise her; few will approach her despite her beauty. She is described as having a reputation that intimidates even teachers. Within the Program, she is instantly active and remorseless. Her assigned weapon is a kama (sickle), which she wields with lethal skill, and she also acquires and uses a pistol. Her primary method is to feign vulnerability, lure male classmates with her sexuality, and then kill them when they let their guard down. She kills six students in the film adaptation and more in the novel and manga, making her the female participant with the highest body count.

Mitsuko has no real friends; her relationships are entirely transactional. She interacts with Hirono and Yoshimi as followers rather than equals. Her encounters with male students such as Yuichiro Takiguchi and Hiroki Sugimura are marked by manipulation and violence. She tries to seduce Sugimura but fails because he is devoted to Kayoko Kotohiki. Her most significant confrontation is with Kazuo Kiriyama, the emotionless killer. After being wounded, she attempts to seduce him, stripping naked and offering herself, but Kiriyama shows no interest and shoots her repeatedly. In her final moments, she reverts to a childlike state, crying out for her father and clinging to the ring he gave her. She is killed by a shot to the face.

Across the story, Mitsuko’s arc illustrates how a victim can become a predator. She operates from a philosophy of pure self-preservation: she has been on the receiving end of cruelty, so she now takes whatever she can. Her death is both brutal and tragic, as it underscores that her manipulative skills are useless against someone utterly devoid of human emotion. She is a character defined by her trauma, her calculated cruelty, and a brief, desperate glimpse of the broken child she once was.