Another one of Sakuya's best friends. She is very beautiful, and often argues with uuūri. Though her name is "Hijiri", she is referred to as "Sei" by Sakuya because the kanji in her name can be alternatively pronounced as such. She is also apparently an intimidating person, and even overwhelms the majority of her teachers. In the ninth chapter of the manga, Yuuri remarks that Chihiro seemed like a second Hijiri. She is the president of her class. Wikipedia

Spoiler

After knowing Chihiro's tragic past, she suggested Shiina not to be with Chihiro because (according to chapter 33) she is afraid that Chihiro will abandon Sakuya and return to Sakura. From chapter 34, the manga revealed that Hijiri's crush was Shizuka, a math teacher and her homeroom teacher since first grade of high school. From chapter 43, the manga revealed that Hijiri and Saki, a man serving her, likes each other. She married Saki in the end.