Keiko Mitarai serves as student council president at her high school, publicly embodying an ideal figure with strict discipline, formidable leadership, and an impeccable reputation that earned her the moniker "demon chairman" for her uncompromising approach. Privately, she maintains a longstanding incestuous relationship with her younger brother, Noboru Mitarai, which began in childhood from mutual sexual curiosity and matured into consistent romantic and sexual involvement. Their encounters occur primarily during their parents' frequent business trips. Keiko exhibits dedicated enthusiasm for fellatio, extensively practicing and refining techniques with Noboru. This preoccupation defines her private behavior. The siblings engage in sexual activities across multiple settings—their home, public spaces like parks and restaurants, and unconventional locations—heightening Keiko's excitement. She often dominates their dynamic, initiating encounters and seeking novel experiences, including the use of parental-provided pills that temporarily revert their bodies to childhood for sexual exploration. Documented activities also encompass erotic feeding, anal sex, and watersports, the latter diminishing observer appeal. Physically, Keiko possesses waist-length brown hair, blue eyes, and a developed figure, frequently accessorizing with a hair clip. She deliberately wears school uniforms showcasing "Zettai Ryouiki" during dates with Noboru.