Izuku is the prince of an empire, and the prince must have a beast servant, beast servant is always beside the prince to protect his master from danger and help with his affairs. Beast knows an Ajin (half-beast human). Some Ajin have powers and some haven't, beast servants for the prince are required to be male and have powers. But the prince's first beast servant died because he was killed... Many of the beasts don't want to nominate themselves to be the prince Izuku's beast attendant.