Tsuki Yadoru

Naoya Amamiya is suddenly adopted into Keiji Mogami's family, which highly prizes martial arts prowess. Naoya is several years older, and therefore better at swordcraft than Keiji, but Keiji's mother pushes him to work hard in order to beat Naoya. Naoya himself seems nice at first, but then he begins to show Keiji a much, much darker side.