The Disturbing Truth Behind Deku's Quirk In My Hero Academia
"My Hero Academia" turns the concept of comic book superpowers on its head thanks to the introduction of Quirks. In this series' world, approximately 80% of the Earth's population is born with some type of power. It sets up many of "My Hero Academia's" characters on their respective paths of morality. Fortunately, series creator Kōhei Horikoshi makes these Quirks balanced and more interesting because they all have their own unique drawbacks.
Series protagonist Izuku Midoriya, who goes by the hero name Deku, is no different. Originally born without a Quirk, All Might transfers the quirk One For All into Deku. However, as this CBR article highlights, Deku is forced to use All For One at low percentages for much of the series because if he doesn't, it will literally break his body. He's already shattered bones numerous times while using his Quirk to try to save some of his friends. While it's likely that Deku will eventually gain total control over One For All, the toil it currently takes on his body isn't even the most disturbing part. That's reserved for its relation to the series' main villain, All For One.
Deku's Quirk only exists because of All For One
All For One is known in "My Hero Academia" as the archnemesis of All Might and one of the most powerful, thanks to his Quirk of stealing other people's Quirks and using their powers for his own. In addition, All For One is also the main cause of One For All's existence, which seems to be the only Quirk that can truly stand against him. In "My Hero Academia" Manga Chapter 59, All Might breaks down the origin of Deku's Quirk and its relation to the villain.
In that chapter, All Might explains that All For One gave his seemingly Quirkless younger brother, Yoichi Shigaraki, an ability to store power within his body. However, Shigaraki contains his own unique Quirk, which was the ability to transfer his Quirk to another person. His own Quirk and the one given to him from All For One merged together to create One For All. Yoichi tried to fight against his brother, but ultimately lost, so he passed the Quirk down to future generations in the hopes that they would be able to stop All For One with it. Deku is currently the ninth user of this Quirk, and it's become immensely stronger over generations.
If All For One hadn't given his younger brother another Quirk, there would be no One For All. But as All Might remarks in the same chapter, its origin is fitting because "Justice is always born from evil."