The Definition of Insanity

During the dawn of the age of quirks, All for One wiped his younger brother's memories and de-aged him. Although Yoichi's friends succeeded in rescuing him, whenever he reached age fifteen, he regressed to infanthood again. With no way to cure him, they hid him from his demented older brother using a series of caretakers. The current incarnation is named Izuku Midoriya.