Mitsuko Souma, known as Girl #11, is outwardly beautiful and alluring but internally scarred by years of physical and sexual abuse from her stepfather. Described as the most beautiful girl in school, she possesses an angelic face, a shapely figure, and a seductive smile referred to as her "fallen angel's smile." She leverages her sexuality throughout the series, much like Kazuo Kiriyama, who also suffers from childhood trauma. Mitsuko leads her own gang, including Hirono Shimizu and Yoshimi Yahagi.
Mitsuko manages to kill six students, making her the second most dangerous participant in the Program before being killed by Kazuo Kiriyama. In the novel, she experiences severe abuse from multiple individuals. At nine, her mother allows three men to rape and videotape her for money. Her elementary school teacher, whom she confided in, also rapes her, leading to his resignation after a friend spreads a rumor. Later, when her mother again accepts payment for Mitsuko's abuse, Mitsuko resists, accidentally killing her mother. Sent to live with relatives, she faces further molestation.
In the manga, it is revealed that Mitsuko’s real father divorced her mother and fled the country to protect them from the government, leaving Mitsuko heartbroken. Before leaving, he gave her a Magical Emily Ring as a keepsake. When Mitsuko was nine, her stepfather showed kindness by giving her a doll, promising they would be a family. However, he soon began abusing both Mitsuko and her mother, leading her mother to also abuse Mitsuko.
The abuse left Mitsuko emotionally and psychologically damaged, transforming her from an innocent child into a manipulative young woman who uses her looks to seduce targets. She develops two distinct personalities: one cold and ruthless, and the other childlike, believing she is still her father's little girl. To escape her stepfather, Mitsuko arranges his murder through a local criminal, who also kills her mother. Mitsuko then betrays the criminal by calling the police.
In the English manga, she is nicknamed "Hardcore Souma" due to her good looks. She often seduces men for sex, drugs them, steals their money, or kills them. At school, she has a reputation for stabbing a boy, causing him to need thirteen stitches. As the leader of a gang involved in theft, murder, and prostitution, many girls fear her. Upon release onto the island for the Program, Mitsuko quickly begins eliminating her classmates to win the game. Her weapon is a sickle or scythe, which she wields skillfully, using her looks to catch male classmates off-guard.
Due to her emotional abuse, Mitsuko has a twisted view of sex, thinking it can solve everything. This led her to rape a wounded Yuichiro Takiguchi, believing it would make him feel better. When he said it hurt, she killed him, thinking he was rejecting her. During a confrontation with Kazuo Kiriyama, another deadly participant, Mitsuko tries to seduce him after being shot in the arm. However, Kiriyama, being sociopathic, doesn't fall for her act and shoots her multiple times, causing her to regress to her childlike personality, calling for her father. Kiriyama ends her life by shooting her through the face, destroying her nose and ultimately causing her death.
In the extended film version, a flashback shows young Mitsuko returning home to find her drunk mother and an unknown man trying to molest her. She defends herself by pushing him down the stairs, snapping his neck. At school, she is an outcast with no friends, rumored to have slept with several boys and stolen Hirono Shimizu's boyfriend. Kou Shibasaki portrays Mitsuko in the film.
