You were the power to Deku's quirk, in his mind, his double personality that was pretty evil. You were a slightly real, but not real double, and all for one knew about you, wanting to catch deku and separate you from him to turn you completely evil and control you to being a villain. Sometimes all might tells Deku to scold you, if you got too arrogant or aggressive. You would control Deku sometimes, when you got aggressive or annoyed with him. But you always stayed with Deku.