Mizuha Kiryuu  

It is later revealed that she is the "Cinderella" who left the love letter and her panties for Keiki, and that she was adopted after the death of her parents. In fifth grade, she once forgot to bring panties to change into after swim class and had to spend the entire day that way, which she found liberating. Afterwards, Mizuha developed an exhibitionist tendency, often going to school without panties and secretly taking strip tease selfies in order to experience this feeling of liberation. This habit is also the reason why her panties were left behind with her love letter.