Satoru Takagi was Tomie Kawakami's high school teacher who engaged in an inappropriate sexual relationship with her while she was his student. During a mountain field trip, Tomie privately informed Takagi of her supposed pregnancy near a cliff edge, seeking commitment. Witnessed by her boyfriend Yamamoto, this led to a confrontation where Yamamoto struck Tomie, causing her accidental fall and death. Takagi convinced students to conceal the death as an accident to protect Yamamoto. He instructed male students to dismember Tomie's body into 42 pieces for disposal, personally ensuring her final death by stabbing her with scissors when she briefly regained consciousness. Tomie's friend Reiko disposed of her heart by throwing it from a bridge. After Tomie's apparent return to school, Takagi confronted her about her identity. Tomie denied being an impostor and whispered information about their past affair, triggering Takagi's severe mental breakdown and institutionalization. Medical staff documented his claims that Tomie was "never a normal human," indicating his deteriorating mental state and exposure to her supernatural nature. He later escaped the psychiatric facility. Following his escape, Takagi transformed into Tomie's active collaborator, facilitating her regeneration experiments and spreading her influence. In the "Morita Hospital" arc, he posed as Tomie's father "Hanako" to authorize transplanting her kidney into critically ill patient Yukiko. This caused Tomie's cells to regenerate inside Yukiko, creating a new Tomie entity. Takagi later infiltrated the hospital to destroy other Tomie cell specimens, burning the basement containing a stored clone. His involvement deepened as he implanted Tomie's cells into an elderly man's daughter, transforming her into a monstrous entity, and later assumed control of the man's property. He trafficked Tomie's biological material, selling "Tomie seeds" to a town. A thematic connection exists between Takagi and a young boy named Satoru from the "Boy" chapter. This boy encountered a regenerating Tomie in a cave where she manipulated him into stealing, forced physical intimacy, and convinced him she was his mother. After witnessing Tomie with another man, the boy murdered the man under her influence. Tomie rejected him, exacerbating his instability, and later seduced his father before abandoning the boy. The story implies this boy became a violent criminal executed as an adult, though it remains unconfirmed if he is the same person as Takagi. Takagi's later activities focused on aiding Tomie's proliferation through scientific means. His ultimate fate is not detailed.