The self-proclaimed childhood friend of Masato Kataoka who affectionately calls him "Maa-kun." Yuki strives to live like a girl, and moves to Tokyo with the hope that he can make a fresh start with his new life. He moves into Masato's apartment and begins attending Masato's high school wearing a girl's uniform. Yuki quickly becomes popular among his peers, and no one is aware that he is biologically male. Even the teacher, Mizutani-sensei, who begins living with Yuki and Masato, does not realize it.
Although claiming that he is not a homosexual or bisexual, Yuki bluntly states that he is in love with Masato because of how kind he was to him when they were kids. He constantly wears a Hello Kitty pendant that he claims Masato gave to him, and reminisces about their childhood together often.
Through out the stroy Yuki transitions from the identity of a crossdresser to a trans girl, despite being bilogically male there is no doubt she is female. The "Maa-kun" that he holds so dearly to his heart is not actually Masato Kataoka, but rather Masato's girlfriend Kaoru. As a child, Yuki mistook Kaoru for a boy due to her tomboyish appearance and behavior.