Kokushi Musou!!

A martial arts hopeful discovers a boy who has been imprisoned for ten years on a hidden island where newcomers are forbidden to leave for a decade. The island draws people seeking to master their arts, as well as children sent by families with martial arts traditions, and its departure contract is strictly enforced. Ro arrives hoping to improve his skills, but feels he is only being held back by less capable peers. When he opens a door in the island’s forbidden area, he finds the boy, who remembers little of his life before his confinement. Ro names him Kid, and Kid sets out to leave the complex and search for his father. With help from Ro and new friends, including Mugi, a young girl who is bullied, he may be able to begin that search.