Sayoko Kikuchi is Koichi Uehara's classmate, enduring relentless bullying from peers who target her with cruel remarks about her appearance, often calling her "ugly." Initially succumbing to peer pressure, Koichi joins in teasing her.

Her parents' divorce prompts her relocation away from the school and neighborhood. Before moving, she encounters Koichi once more. During Coo the kappa's departure from the Uehara family, Koichi shows her Coo as a gesture of kindness. The two pledge to stay in touch through letters after her move.

Sayoko's arc underscores themes of bullying and isolation. Koichi's shift from reluctant participation in her torment to defending her marks his personal growth, influenced significantly by his time with Coo. Details regarding her family life beyond the divorce or her experiences after relocating remain unexplored.