Dekiru

Ryan McFall, an orphaned boy, is shipwrecked on an island with another boy. The two are raised and trained in martial arts by the island's sole inhabitant. After Ryan is beaten unconscious by the other boy, he awakes to find himself back on his home shore with no memory of his past. He is adopted by the family who discovers him and they nurse him back to health. Ryan tries to lead a normal life - although he and his family come to realize he possesses superhuman abilities and they try to keep his secret hidden from the rest of the world.