WARNING: The following contains spoilers for My Hero Academia Chapter 287, "Mistake” by Horikoshi Kohei, Caleb Cook, and John Hunt, available now in English from Viz Media.
Since My Hero Academia's very beginning, Shigaraki Tomura has been the story's primary antagonist. Using the combination of his fearsome Quirk and tactical expertise, Shigaraki climbed his way to the top spot of Japan's most dangerous villains in a matter of months. Indeed, the whole world seems ripe for his taking -- but his plans for total destruction may be getting foiled by his own mentor, All For One.
As a child, Shimura Tenko's sudden Quirk manifestation -- as well as hidden animosity toward his family members -- caused him to murder all of them with Decay. All For One found him roaming the streets lost, with nowhere to go. The villain leader took young Shimura in, promising him a better life where he could embrace his every impulse without fear of restriction or consequence. As a show of affection to his new "son", All For One gave Shimura a new name, Shigaraki, with the first name Tomura, which means "to mourn".
All For One nurtured Shigaraki's worst instincts for a singular purpose in My Hero Academia -- stealing One For All. His previous attempts at snatching the power-stocking Quirk had failed as his will wasn't greater than those of the vestiges hidden within OFA. Therefore, he taught Shigaraki to hate the hero society and raised him to believe his destiny was to destroy it. For All For One's plan to work, he needed a very strong-willed person. Luckily, Shigaraki's tragic childhood gave him an excellent base to start with, never revealing that the boy was just another pawn.
Being the central piece of a plan decades in the making didn't mean Shigaraki's quest for power was easy. After All For One was captured, Shigaraki had to prove he was worthy of his master's power to Dr. Garaki. After doing so, he was granted the All For One Quirk -- but the power came with a secret. Just as One For All stored the consciousness of each of its previous wielders, All For One did the same with all of the Quirks stored within it. Its original user's consciousness was the strongest, and Shigaraki found himself locked in a mental battle with his master for control of his body.
Instilling Shigaraki with such a strong will turned out to be a double-edged sword, however, as taking control of his ward's body didn't go as smoothly as All For One planned. He faced fierce resistance from Shigaraki's consciousness, which was unwilling to let go of his dream of carrying out total destruction himself. Their two consciousnesses appear to meld together while they face outside threats, meaning their internal conflict hasn't made Shigaraki's body any less dangerous. Yet in their moments of solitude, it is clear that All For One has the upper hand in their struggle.
Having visited their vestige realm before, My Hero Academia protagonist Midoriya Izuku is aware of the dilemma Shigaraki currently faces. Despite the many atrocities the villain has committed, Midoriya has made the very interesting decision to save him from All For One's clutches.
His good-natured resolution was quite unpopular among the previous One For All users when he informed them about it, but it makes sense given the revelations Midoriya has had about how the hero society treats non-conformists. Given that Shigaraki is still desperately fighting All For One for control, Midoriya's quest isn't an exercise in futility.
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