Sonata wa Ittai Dare nandaソナタはいったい誰なんだ; Nobody Knows About the Hero Sonata; Кто такой Соната?

An "unknown boy" has defeated the Demon King and saved the world. The whole world waits to learn of who he is, but it turns out the boy himself has forgotten everything. Then two people appear before him, both claiming to be his "family".