Anitomo.

Sota Nishino is a good-looking high-school student. He meets Mai, who is his friend Yukihiro Nanase's younger sister. She is also a high-school student and somewhat naïve. Sota Nishino mutters to Yukihiro Nanase, "Your sister is cute." Mai overhears Sota's remark through the thin wall between her room and her brother's.