The world's greatest Hero, All Might's real Quirk is unknown to the public, but he passes it on to Izuku after seeing the boy's bravery.
The world's greatest Hero, All Might's real Quirk is unknown to the public and most people concludes that his Quirk is some sort of strength-enhancing Quirk. He passes his quirk on to Izuku after seeing the boy's bravery.