Nagisa Mizuki, her surname combining the kanji for "water" (水) and "city" (城), her given name meaning "shore" (渚), stands as a central narrative figure. Initially portrayed as a student engaging in severe bullying of Satou Futoshi, her actions encompass humiliation and psychological torture. This conduct reflects a personality described as fundamentally "evil," possessing a natural inclination toward cruelty, evident when subconscious insults during hypnosis reveal her genuine contempt. She deliberately maintains a public facade of kindness and sweetness, manufactured to secure social approval, contrasting with authentically kind characters.

She demonstrates loyalty within her relationship with her womanizing boyfriend, Akira Aihara, refraining from infidelity. Nagisa places significant personal value on preserving her first kiss and virginity for marriage, indicating forethought regarding her physical autonomy. This resolve is violently undermined when Satou Futoshi uses hypnosis to manipulate her into believing he is Akira. Tricked into kissing him and changing into her swimsuit, she realizes the deception. Satou then employs hypnosis to rape her, an assault repeated later under similar circumstances.

Following these traumatic events, Nagisa develops a complex psychological dependency on Satou, characterized by simultaneous lust and fear. This drives her to confront Shouko Aihara, interrogating her about her connection to Satou. During this encounter, she discovers Shouko's intersex biology and proceeds to violate Shouko's body. Satou interrupts this torment, scolds Nagisa, and subjects both her and Shouko to further sexual violence.

The character's background remains confined to these events within the documented material.