In recent chapters, the backstory of Vigilantes' main villain, Number Six, has been explored, showing how as a child, he was experimented on by All for One and his henchman Dr. Garaki.
It's been known for a while that All for One's Nomu are made from ordinary people, often those who've just died, which may be the case for Six. All for One provided him with a quirk, and carefully conditioned the boy to envision the quirk's previous owner, O'Clock, in his mind, providing advice.
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