Reiko is Tomie Kawakami's childhood friend and classmate. She witnesses Tomie fall to her death from a cliff during a school trip after an altercation with her boyfriend Yamamoto and teacher Mr. Takagi. Reiko participates in the subsequent cover-up, standing guard while Tomie's body is dismembered. She disposes of Tomie's heart by dropping it from a bridge into a ravine. After Tomie's funeral, Reiko is horrified when Tomie reappears at school. Crushed by guilt and fear, Reiko tells Yamamoto of their plan to confess to the police. Their classmates aggressively pursue them. Confronted, Tomie questions Reiko about her closeness to Yamamoto. Reiko denies any connection and flees in terror, abandoning Yamamoto, who is later found irreversibly insane. Reiko's family relocates to a seaside town. Reflecting on the events, she notes classmates dropping out or dying by suicide. She questions whether the incident was a mass hallucination. Later, walking on the beach near her new home, she stumbles upon the empty wrapper that once held Tomie's heart. This discovery leads her to a nearby sea cave, where she observes a regenerating entity resembling Tomie, confirming the horrifying reality of Tomie's return. Reiko's character arc concludes with her relocation and ongoing psychological trauma.