Tsutsukakushi Tsukushi♀

A third-year student, Tsukiko's older sister and the president of the track and field club. She is called "Steel King" because of her strict and serious personality and was the one who made Youto known as the "Perverted Prince". She is deeply in love with Tsukiko, to the point that she wants to marry her despite their distant relationship, and wants to move to Massachusetts where same-sex marriage is allowed. She is actually very airheaded, shown how when she confronted Youto about his relationship with Tsukiko, she believed his lie that the person associating with Tsukiko is his "younger identical twin".

[Edited from Wikipedia]