Yuuzuki Kiryuu

She is Mikazuki's older sister, the new teacher to Gakushū Senior High and homeroom teacher to Keita and Miharu, who in an earlier chapter was a clothing store clerk because she liked western style clothing. Through bad timing and unaware that Keita and his sisters are not related by blood, she believes that he is a deviant who dates multiple girls while having an incestuous relationship with his sisters. She becomes excited by her exaggerated ideas of what he and his sisters do at home or with other girls. Through these beliefs, she tries to recruit Miharu, believing she is one of the girls Keita is dating, to have them go on a date to separate him from his sisters. She is a virgin, however after clearing everything up, she starts to have growing feelings for Keita as well that goes to a lustful desire, practically raping his P.E. clothes as she is addicted to the scent of his sweat and fantasizing them having sex. She is a closet fujoshi (female otaku) with a samurai fetish, and also likes to cosplay. Due to this, she spends so much money on otaku goods that she usually eats instant noodles. This is also why she no longer lives with her parents, as her mother would disown her if she continued with her otaku ways. The sisters see her as a possible threat for Keita's heart because they believe he has fetish for older woman, and the fact that she has a much bigger bust than them. She also mistakenly believes because of all of the outfits the twins usually wear solely to get Keita's attention that they might be just like her. In the anime, she is already the twin's homeroom teacher before Keita graduated.

(Source: Wikipedia)