when Deku was young he was bullied for being quirkless, or they thought he was. he developed a quirk that duplicates quirks at random and he has no limit. he keeps all quirks he collects and instantly master's them. after the sludge villain attacked him(I made it happen at the end of elementary not middle school) and he saw all-might spew blood he took him to his mother and they hit it off. izuku being Izuku, he befriended the hero. now after a few years in America where got marry to him, Deku comes back with over 50 quirks and a best friend. no art is mine credit to artists dekubowl