Girl #11, Mitsuko Souma is beautiful and seductive on the outside yet inside, she's a hollow shell, having been left damaged after suffering years of physical and sexual abuse from her stepfather. She is said to be the most beautiful girl in school with her angelic face, shapely body, and seductive smile which is referred to in the novel as her "fallen angel's smile." She uses her sexuality to her advantage throughout the series. Similar to Kazuo Kiriyama, Mitsuko suffers from childhood trauma, and she is the leader of her own gang, which consists of Hirono Shimizu and Yoshimi Yahagi.

She manages to kill a total of six students, becoming the second most dangerous opponent in the Program, before being killed by Kazuo Kiriyama.
In the novel, Mitsuko is abused by several individuals. When she was nine years old, her mother was paid to let three men rape and videotape her. Next, she was raped by her trusted elementary school teacher when she told him of her previous rape. A friend of hers witnesses the rape and spreads a rumor which leads to the teacher's resignation. Later, Mitsuko's mother again accepted payment to let Mitsuko be raped but Mitsuko resisted, accidentally killing her mother in the process. She is sent to live with relatives, only to be molested once again by the father of the household.

In the manga, it was revealed that as a child, Mitsuko's real father divorced his wife and eventually fled the country altogether, intending to protect her and young Mitsuko from the government. This left Mitsuko completely heartbroken and in tears. Before he left, her father gave her a Magical Emily Ring because he knew that she wanted to have one and also so that she could have something to remember him by. Mitsuko would later treasure the Ring and use it as a possible source of comfort even during the Program.

Some time later, and when Mitsuko was nine years old, she then met her stepfather who ended up giving her a doll which was the only time he'd ever really shown her kindness along with the promise that they'd all be a family. Shortly after that, things changed as he then began subjecting both Mitsuko and her mother to all forms of abuse. In time, the damage was too much for even Mitsuko's mother to take and she too began abusing her own young daughter.

Eventually, the horrific abuse Mitsuko had endured during her childhood ended up leaving her physically, emotionally and psychologically damaged for life and she went from being a once happy, innocent child to a manipulative young woman who used her good looks and body to seduce her male targets while also giving her the delusion that touching seemingly makes it better, something that her stepfather regularly told her while abusing her. The resulting trauma also caused her to develop two completely separate personalities: the cold, ruthless Mitsuko who is extremely manipulative and deprived of mercy whenever it comes to killing her classmates or ruining the lives of others to get what she wants and the young childlike Mitsuko who still believes that she's Daddy's girl and begs for him not to leave because he promised that he'd stay with her.

Growing tired of her stepfather, Mitsuko later made a deal with a local criminal, arranging for her stepfather's murder, but he ended up killing her mother as well. Mitsuko then betrayed the criminal by calling the police on him. In the English manga, she's nicknamed "Hardcore Souma" presumably due to her good looks. She uses much of her sexuality in the manga and often seduces men before having sex with them and later ends up either drugging them and stealing their money or just killing them which she began doing during her tenure on the Program.

She developed a reputation in school, including when she stabbed a boy, causing him to get thirteen stitches. Souma is head of a gang at school, where she participates in theft, murder, and prostitution. Many of the girls fear Mitsuko in the manga. Almost immediately upon being released onto the island for the Program, Mitsuko wastes no time in finding and killing her classmates one by one as she wants to win the game. Her designated weapon is a sickle or a scythe which she wields with incredibly lethal skill and she also uses her good looks and body to seduce her male classmates so that she can catch them off-guard.

Due to her emotional abuse, she has a twisted idea of sex, thinking that it is the solution to everything, which led her to rape a wounded Yuichiro Takiguchi, thinking it would make him feel better. She became upset when he said that it hurt him, and thinking that he was rejecting her, she killed him. She later came across Kazuo Kiriyama, another student who similarly to Mitsuko had been responsible for the majority of the deaths during the Program and almost immediately, the two engaged in a gunfight. Having been shot in her right arm, Mitsuko then attempted to seduce him by using her good looks and body, a tactic she'd used on other men in the past. However due to his sociopathic behavior, Kiriyama didn't fall for her act and proceeded to injure Mitsuko by shooting her in random parts of her body which left her in agony and caused her to remember various moments of her life. Her childlike personality seizing control, Mitsuko began calling for her father while saying that she was Daddy's girl. Kiriyama then proceeded to end Mitsuko's life by shooting her through the face which destroyed her nose and eventually resulted in her death. It should also be noted that due to the sexual abuse she was subjected to as a child, Mitsuko is shown to have misandry, a hatred of boys and men.

In the extended version of the film, a flashback portrayed a young Mitsuko returning home from school. At home she found her mother drunk, and an unknown man who then tried to molest her. Mitsuko retaliated in self-defense and pushed him down the stairs, snapping his neck and killing him. She is an outcast in school and has no friends. It was rumored that she slept with a few guys at school and stole Hirono Shimizu's boyfriend. She is played by Kou Shibasaki in the film.

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