Younger sister Kiryuu Keiki. A sweet and cheerful girl. She cares a lot about her brother and gives him various advice. As the story progresses, she joins the calligraphy club, trying to seduce Keiki before the other members do. She turns out to be a perverted exhibitionist who likes to undress in her brother's room and go to school without her underwear. It is also later revealed that Mizuha is not a blood relative of Keiki. Moreover, Mizuha is the very "Cinderella" that Keiki was looking for.
