My Hero Academia's big climactic battle of the War Arc has taken a strange introspective turn. As the powers of One For All and All For One keep erupting from Izuku Midoriya and Tomura Shigaraki (respectively), the two boys have found themselves both consumed by their powers. More specifically, Deku and Shigaraki have both found themselves caught up in the influence of the previous holders of One For All / All For One, with those souls waging their own battle on the mental/spiritual plane of existence. It's in that spiritual battle that we learn a stunning new fact about My Hero Academia's quirks!
Warning! My Hero Academia Manga Chapter 287 SPOILERS Follow!
On the mental/spiritual plane (or whatever you wish to call it), Tomura Shigaraki is being held hostage by All For One's original user, who has hijacked his protege's failing body in an attempt to steal Izuku's One For All power. Deku is meanwhile being guarded by One For All's previous users, led by Nana Shimura, Shigaraki's superhero grandmother. The ever-boastful All For One can't help seeing his former victim and not sharing some knowledge about quirks he learned in his work with mad doctor Garaki (aka Ujiko):
"Just as organs and cells house memories of sorts, every quirk factor contains a consciousness - the individuality of its wielder, if you will."
That key new dynamic about quirks isn't just left to hang in the open air for interpretation; All For One quickly explains just how the quirk-consciousness connection makes the All For One and One For All powers truly something unique:
"My blood kin too - my little brother who had the power to grant his own quirk to another, could also meddle with quirks directly. When his original quirk mixed with he power-stocking one, the resulting power became a sleeper car to carry consciousness forward to future generations."
This revelation about quirks and their connection to All For One/One For All brings so many things about My Hero Academia's core mythology into clearer focus. It explains why One For All comes with the consciousnesses of its previous users, while also opening the door to some dark new implications of what All For One's power is all about. For instance: does the villain hold any sway over people whose quirks he's stolen? If so, there are potential unwitting sleeper agents embedded in the heroes' midst...
There's also the mystery of what the consciousness of One For All's 4th user is about (in addition to his quirk power). That mysterious person could be a key personality that has yet to emerge - and the knowledge that consciousness brings could be major salvation - or totally devastating.