NAKAIMO My Little Sister Is Among Them!: Season 1

Shogo Mikadono is the son of an industrial magnate named Kumagoro Mikadono who passed away. As a final wish, Kumagoro imposed a condition Shogo must meet before he can inherit the family legacy—find a female mate at school. Shogo transfers into a school to fulfill that condition. However, he learns that he has a younger sister from whom he was separated at birth. He does not know what his long-lost sister looks like, but she happens to be one of the students at his new school.