The power to Deku's quirk, in his mind, his double personality that was pretty evil. The personality was slightly real, but not real double, and all for one knew about it, wanting to catch deku and separate it from him to turn it completely evil and control it to being a villain. Sometimes you would be able to scold the personality, if it got too arrogant or aggressive, or calm it down. The second personality would control Deku sometimes, when it got aggressive or annoyed with him.