While My Hero Academia season 5 details how antagonist Tomura Shigaraki (Eric Vale) first manifested his quirk Decay, one theory suggests that the quirk actually belonged to Deku first. During Shigaraki’s fight with Re-Destro in Deika City in episode 23 "Tenko Shimura: Origin," My Hero Academia finally reveals through a series of disturbing flashbacks the events of Shigaraki’s childhood that catalyzed his journey to becoming All For One’s successor, but the coincidental nature of his tragic origins seem to signify a more sinister conspiracy happening behind the scenes.
Unlike past seasons of My Hero Academia, which follow the perspective of Izuku Midoriya (also known by his hero name Deku) as he trains to become a professional hero at U.A. High School, the second half of My Hero Academia season 5's story provides more insight into the League of Villain's backstories and their conflict with the Meta Liberation Army, a villain organization that believes civilians should have the right to use their quirks freely. After Shigaraki defeats their leader Re-Destro in combat, Re-Destro elects to make him their new Grand Commander after viewing how he embodies their ideology of quirk liberation, making the League of Villains go from small-time criminals to a legitimate threat to superhuman society overnight with the support and resources of the Meta Liberation Army.
Despite the fact that the plot shifted to focus more on the villains, My Hero Academia season 5 draws a lot of parallels between Deku and Shigaraki’s origins. In episode 23, it’s revealed that Shigaraki admired All Might as a child and dreamed of becoming a My Hero Academia hero like Deku. Within the episodes that depict Deku and Shigaraki's separate childhoods, they both describe being misunderstood and wish their family members had shown them unconditional support. Even Shigaraki’s battle against Re-Destro in season 5 is reminiscent of Deku’s fight against Overhaul in season 4, since both conflicts resulted in Deku and Shigaraki increasing the range of their abilities. The comparisons between Deku and Shigaraki in My Hero Academia season 5 might’ve been a way of emphasizing that the two have more in common than they once thought, including the quirk Decay, the ability to turn whatever one touches to dust.
Shigaraki’s Origins Suggest All For One’s Involvement
While Shigaraki casually compares the manifestation of his quirk in My Hero Academia season 5, episode 23 to a villain "attacking" their family, that supposition might not be that far away from the truth. Given the fact that the deaths of Shigaraki’s entire family coincidentally benefitted My Hero Academia's All For One the most, it suggests he may have orchestrated Shigaraki's tragic past. After being reprimanded by his abusive father for discovering a picture of their estranged grandmother (Nana Shimura) in his father’s study, Shigaraki suddenly manifests his quirk Decay at the unnatural age of five, which causes him to accidentally turn his entire family to dust. After wandering the streets alone in search of help, Shigaraki is taken in by All For One, who mentions how unfortunate it was that a hero never came to save him. Since the manifestation of Shigaraki's quirk resulted in him being left vulnerable and alone with a traumatizing dislike of heroes, it’s possible that All For One alternatively gave Shigaraki a quirk with the aim of acquiring him as an apprentice.
While the arch-villain All For One has been a mysterious figure throughout the My Hero Academia anime movies and show, he’s known to be very meticulous and manipulative. During his prime, All For One assembled followers to his cause by sympathetically offering to give or take away quirks in exchange for their loyalty. He even insinuates during his final battle with All Might at Kamino Ward that he intentionally recruited Shigaraki as his successor because he knew Shigaraki’s relation to All Might’s former master Shimura would torment All Might. Since All For One’s own quirk allows him to transfer powers from one person to another, it’s within the realm of possibility that All For One secretly bestowed Shigaraki with a destructive quirk that would place him in the position to be exploited. Since most children inherit their quirks in My Hero Academia before the age of four, it’s also rather unlikely, albeit not unheard of, that Shigaraki would naturally manifest a quirk at such a late age.
Deku’s Original Quirk May Have Been Stolen
While Deku was introduced in My Hero Academia season 1 as “quirkless,” or born without an ability, the concept that quirks can be transferred or stolen has since become a common theme throughout the series. In episode 1 “Izuku Midoriya: Origin,” a quirk doctor tells young Izuku that, while he should’ve already developed an ability similar to his parents’ quirks, Izuku’s x-rays suggest he won’t manifest one at all. “When superpowers first began appearing, there were many research studies conducted, and doctors discovered a link between the bones in a person’s foot and their likelihood of developing a quirk,” says the doctor. Since Izuku has two joints in his pinky toe, the doctor concludes that Izuku is amongst the 20% of people in the My Hero Academia universe that hasn’t developed powers. While the doctor’s diagnosis didn’t seem suspect at the time, his striking resemblance to All For One’s mad scientist Dr. Ujiko suggests something more nefarious.
In chapter 258 of the My Hero Academia manga, it’s revealed that Dr. Ujiko is actually a renowned public figure in the medical community, a respected quirk doctor named Kyudai Garaki who founded Jaku General Hospital. During the heroes' assault against the Meta Liberation Army, the laboratory where Ujiko creates the High-End Nomu is even discovered to be located in the hospital’s basement. After the revelation of Ujiko’s public persona, a popular fan theory suggested that Ujiko exploited his status as Garaki to not only study his patients’ quirks, but also to conduct experiments on them or even steal the quirks with the most potential, which Ujiko stockpiled for All For One and My Hero Academia's League of Villains. While Garaki and Izuku's meeting in My Hero Academia episode 1 might've been a coincidence given Garaki's standing as a local quirk doctor, Izuku alternatively could've been one of Garaki's victims who was born with a powerful quirk that Garaki stole for his own purposes and later covered up by diagnosing Deku as quirkless.
Theory: All For One Gave Shigaraki Deku’s Quirk
If Dr. Garaki specifically stole Deku’s quirk from him as a child because of its impressive potential, it’s also likely that same quirk could’ve been the ability that All For One chose to pass down to his quirkless successor, Shigaraki. In the My Hero Academia universe, children often inherit one of their parents’ abilities or powers from a combination of both. According to Deku’s mother, the inherited quirk Deku should’ve naturally received would’ve either been his father’s quirk, breathing fire, or her own ability to levitate small objects. While it’s unlikely that either of those abilities resulted in the quirk Decay, Deku still could’ve manifested it as a result of a mutation, similar to Eri’s quirk Rewind. The timing also fits. While Shigaraki is five years older than Deku, Shigaraki didn’t manifest Decay until age five, leaving roughly a year for Deku to manifest the quirk and have it stolen by Dr. Garaki.
If the theory is correct, it could have devastating consequences for Deku. Since All For One is a very methodical villain who had the foresight to recruit Shimura’s grandson as his successor with the sole aim of causing All Might pain, stealing Deku’s quirk and giving it to his arch-nemesis might’ve been part of an elaborate plan devised by All For One to harm Deku in the future as My Hero Academia's next wielder of One For All. One of Deku’s greatest strengths as a hero is his infallible sense of self-assurance, which pushes him to work harder and even inspires other heroes to do the same. Since Deku’s main motivation as a hero in My Hero Academia is to save everyone, Deku discovering that his own quirk caused so much death and destruction in the wrong hands could break him and cause him to question his own right to be a hero if his quirk put others in danger.
How The Deku’s Original Quirk Theory Could Spell Shigaraki's Defeat
But at the same time, the theory that Shigaraki wields Deku's original quirk could hold the secret to how Deku will ultimately triumph over the League of Villains and foil All For One's plans. Now it's been confirmed that quirks can be transferred between people, the My Hero Academia anime has also implied and outright revealed that quirks carry traces of their previous users. This is shown to be the case with Deku's Black Whip quirk and would theoretically apply to any other powers All For One has stolen and transferred to other people throughout his evil dealings.
If Deku was the original wielder of Shigaraki's quirk Decay, this would also imply that some trace of Deku is left in the power, which could give My Hero Academia's protagonist an edge over the leader of the League of Villains. It's unclear how exactly this could play out, but it's possible that Deku could exert some control over Decay even while Shigaraki is wielding it, which could be the key to the antagonist's defeat. By the ending of My Hero Academia's anime, if Deku really did wield Decay first, he could learn how to tap into the quirk and gain a unique advantage over Shigaraki that nobody else has.