There's nothing wrong about this. There's nothing wrong about my feelings towards you. Because... we aren't really brother and sister.
— to Keiki Kiryuu
Mizuha is an attractive and cute girl with neck-length light brown hair and blue eyes. She is normally seen wearing her school uniform, but she also wears normal clothes at home.
Mizuha is a kind and cheerful girl who acts somewhat airheaded and cute at times. She is good at giving different types of advice to Keiki Kiryuu and she cares for him dearly that she enjoys being pampered by him.
She appears to be a normal girl who gets along well with her brother, but she is later shown to be deeply in love with him and 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.
It's been noted that Mizuha's actually very popular with the boys at school and has been confessed to multiple times. But she turned them all down.
In the light novel, it is shown that Mizuha is actually not blood-related to Keiki Kiryuu, 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 do so.
- Keiki Kiryuu - Keiki is Mizuha's older step-brother. At first, Mizuha appears to be an ideal younger sister, but she's actually deeply in love and obsessed with him. It's been revealed that she's been in love with him the moment she first met Keiki because he told her to always be by her side (after she's lost her own parents).
- Calligraphy Club - She likes them but doesn't like their approaches to her brother's affection.
- Student Council - Unlike Calligraphy Club members, she didn’t get along well with them deep down since they are the reason Keiki spending his time less and less with her and her clubmates due to him helping them out with their student council works, not to mention that her stress keeps accumulating due to holding back her exhibitionist urge while the club was under their surveillance.
- "I believe that Nii-san should only be spoiled by his little sister."
- "…It’s fine. It’s still fine. There’s no need to panic. I mean, little sister stuff is really popular right now, and I’m not his real little sister either, so we can get married… Yes, the one to win in the end will be me…"
- The name Mizuha means "felicitous omen, auspicious" (瑞) (mizu) and "leaf" (葉) (ha).
- Mizuha's surname Kiryuu means "paulownia" (桐) (kir) and "living" (生) (yuu).