As a character of the original novel series since Volume 5, Hiiragi is a senior of Taro, but her body looks like a girl of preadolescence. She is the president of the Invention Club at Taro's school, and is a genius with an IQ of over 200. Due to her insecurities at not being able to lead a normal life, she comes up with a scheme which would turn everyone in the world into perverts (if they weren't already one). However, she is talked out of it by Taro, and is saved by him when the broken machine threatens to fall on her. She falls for Taro after that incident, and he happens to be her first love (she gave him a mound of lilacs which was, as stated by Arashiko, holds the meaning of first love in the language of flowers.) Like Mio, she dislikes people insulting her childish stature.