Tohru Takamura, a transfer student, temporarily moves in with the Sakumoto family while his mother has surgery. Though not blood relatives, this familial link places him in Manami Sakumoto's home.

An accident upon his arrival leaves Manami drenched; Tohru sees her exposed state and is mistaken for a trespasser, creating an immediate tension. At school, he witnesses Manami being bullied, notably by Chiaki. Attempting to intervene during one confrontation, Tohru falls from a tree and is briefly knocked unconscious. While unconscious, seeing Manami's distress prompts him to call out "Nami," triggering her transformation into a forceful alter ego.

Tohru deduces Manami's dissociative identity disorder and recognizes that saying "Nami" activates this secondary personality. He strategically uses this knowledge to shield her, such as during an incident involving stolen undergarments meant for public shaming. His role shifts from protector to a supportive figure navigating interactions with both Manami and Nami. He develops romantic feelings for both distinct personalities, appreciating the unique traits of each.

As their relationship deepens, Tohru participates in uncovering the origin of Manami's split. He devises a plan aimed at integrating both identities into one cohesive whole, driven by a desire to preserve both personas. This plan involves intimate encounters intended to foster emotional and psychological reconciliation between Manami and Nami, highlighting his commitment to their well-being and deep emotional investment.

His background details remain limited to his temporary relocation due to his mother's health. The narrative centers on his interactions with Manami/Nami and his role in addressing her psychological condition during his stay.