Mizuha Kiryuu (桐生 瑞葉, Kiryū Mizuha) is one of the main characters of the series and a first year student. She is also the little sister of Keiki.
She is revealed to be the 'Cinderella' that Keiki was looking for.
She is an attractive, cute girl with blue eyes and short light brown hair. She is normally seen wearing her school uniform but also wears normal cloths at home.
Mizuha is a kind and cheerful girl and acts somewhat air headed and cute at times. She is good at giving different types of advice to her brother and cares for him dearly. She enjoys being pampered by him.
She appears to be a normal girl who gets along well with her brother, but is later shown to be a pervert as she is an exhibitionist who not only strips in her brother's room, but also does not wear her panties to school on a regular basis.
In the light novel, it is shown that Mizuha is actually not blood-related to Keiki, and with this revelation, she goes on a date with Keiki before it was revealed that she was an exhibitionist.
She later joins the Calligraphy Club in an attempt to seduce Keiki before the other members of the Calligraphy Club does so.