After her father discovers her involvement in an adult video and expels her from home, college graduate Sayoko becomes homeless, finding temporary refuge with her younger cousin Shinichi's family. She has short blonde hair and typically wears a dark green cardigan over a white blouse. During her stay, she frequently teases Shinichi—a fifth-grader—about his emerging romantic relationship with his study partner Mika, his developing sexuality, and physical maturation, including references to his pubic hair. Her interactions with Shinichi occasionally involve suggestive physical proximity, like rubbing against him.

Shinichi and Mika secretly view Sayoko’s adult video, sparking curiosity. This exposure leads Mika, influenced by the content and possibly competing for Shinichi’s attention, to attempt initiating a sexual encounter. Shinichi declines, believing they are unready.

Sayoko eventually moves out, but her presence leaves a lasting impact on Shinichi and Mika’s relationship dynamics. Years later, as second-year junior high students continuing their relationship, the psychological echoes of Sayoko’s influence and adolescent hormonal complexities persistently shape their interactions and emotional landscape.