Yamamoto was Tomie Kawakami's classmate and boyfriend at their high school. While on a mountain field trip, he discovered Tomie's secret sexual relationship with their married teacher, Mr. Takagi. This revelation sparked a violent confrontation where Yamamoto struck Tomie, leading to a struggle that resulted in her fatal fall from a cliff. Initially intending to report the incident and seek medical aid, Yamamoto was persuaded by Mr. Takagi to join the class in concealing Tomie's death. Driven by collective hatred for Tomie, shared by all classmates except her friend Reiko, the group dismembered her corpse into 42 pieces. Yamamoto participated in this act, helping to cut the body, and each student received one piece to dispose of individually. Following Tomie's funeral, she inexplicably returned to school unharmed. Yamamoto, deeply disturbed by her reappearance, avoided all contact. He confided in Reiko, believing Tomie had returned to psychologically torment those involved in her death and its cover-up, fearing she would haunt them until they confessed to the police. When Yamamoto and Reiko resolved to report the crime, their classmates discovered the plan and attempted to murder them to maintain secrecy. Tomie intervened during this attack, inadvertently enabling Yamamoto and Reiko to escape immediate physical harm. However, the cumulative psychological trauma overwhelmed Yamamoto. He suffered a complete mental breakdown and was later found in a state of irreversible insanity.