The following contains spoilers for My Hero Academia Season 6, Episode 10, "The Ones Within Us", now streaming on Crunchyroll, Funimation and Hulu.
In the My Hero Academia universe, over 80% of the population are born with special meta-abilities known as Quirks. Despite how commonplace these powers have become, there’s still very little known about their origin or what their appearance may bode for society in the future -- until now.
All For One, the series' primary antagonist and resident Quirk expert, divulged an unsettling truth about Quirks in Season 6, Episode 10 that explains how his residency in Shigaraki Tomura’s body is possible.
How All For One Knows So Much About My Hero Academia’s Quirks
All For One’s wealth of knowledge concerning Quirks is only to be expected. After all, he and Dr. Garaki have collectively conducted more research on Quirks than any other existing body. His own ability to steal and transfer the Quirks of others necessitated that All For One understood them better than anyone else, lest he fall victim to the side effects of his own. With a goal as ambitious as world domination, the mighty villain was determined to subdue and assimilate any Quirk that threatened him.
For all his expertise at controlling Quirks, it was ironic that a power of his own creation would become the Quirk that frustrated him most. During yet another attempt at stealing One For All from Midoriya Izuku, All For One revealed the truth about Quirk transference in the psychic vestige realm. He failed to take One For All yet again, but before retreating into Shigaraki’s consciousness, offered a theory as to why that Quirk in particular continued to resist him.
Apparently, every Quirk carried some trace of its original wielder's psyche within it. All For One realized that even if it was stolen, a Quirk still retained some vestige of its original owner’s personality. He described this continuity as similar to the phenomenon of organ recipients adopting some similar mannerisms to the original owners after a transplant is completed.
How Did All For One Gain Control of Shigaraki Tomura’s Body?
All For One himself is living proof of this theory’s validity. He shared that he sometimes had nightmares where the consciousness of an original Quirk owner would disparage him, but that he never encountered them again after giving their Quirk away. Being able to perceive and subdue other people’s vestiges is an ability unique to All For One since the natural use of his Quirk forces him to come into contact with the vestiges of many others.
Shigaraki Tomura inherited the All For One Quirk, along with all the other Quirk factors and his master’s extraordinarily active vestige locked within it. Shigaraki has complete control over his body under normal circumstances but, as has already been depicted in the My Hero Academia anime, All For One is capable of wresting control when Shigaraki is especially vulnerable. Their relationship seems to be on equal footing right now, but there’s no telling how long All For One will be happy with this arrangement.