Kotomi Tsuda (津田 コトミ Tsuda Kotomi) is Takatoshi's younger sister, who ultimately ends up attending Ōsai the next year. A cheerful girl, she is considerate and caring toward others, but is curious and enthusiastic about sexual matters. She is very close with her brother, but her comments sometimes imply they have an incestuous relationship, often to Takatoshi's frustration. Nonetheless, she gets along very well with the other girls and is always asking and getting help constantly from them.