Kyōko Hayato is a 27-year-old mathematics teacher at Kagome Academy, an all-girls school. She possesses a tall stature of 1.74 meters, brown hair, and blue eyes. Despite her adult age, she exhibits a somewhat childish demeanor, contributing to her popularity among students. Her expensive tastes, including a preference for luxury dining and driving a sports car (specified as either a Mazda RX-7 or RX-8), lead to chronic financial difficulties, yet she maintains a consistently upbeat attitude.

She develops a romantic and sexual interest in Momoko Naitō, a student serving on the disciplinary committee. Kyōko frequently invites Momoko to spend time with her after school and allows her to stay at her high-rise apartment. During these interactions, Kyōko reveals personal details about her past, including that her first sexual experience occurred with a college student she admired while she herself was still in elementary school. This relationship culminates in a sexual encounter between Kyōko and Momoko.

Shortly after this intimacy, Kyōko collapses due to a severe illness later identified as leukemia. The illness necessitates her immediate resignation from teaching to focus on medical treatment and recuperation, abruptly ending her romantic involvement with Momoko. While hospitalized, she receives a visit from Momoko and another student, Shinobu Handa.

Following her resignation and recovery period, Kyōko travels internationally to Amsterdam to visit a hospitalized friend. She maintains indirect contact with Momoko by sending her a letter describing her experiences during this trip. Her long-term health status and specific medical requirements, including the potential need for a donor, remain unresolved.