In the shonen sensation, My Hero Academia, 80% of the world's population harbors some kind of special ability, called Quirks. These powers may manifest as superhuman strength, exceptional intellect or something else entirely; quirks are as diverse as the characters who own them. The 20% of people who don't have one, however, are alienated in a society that arguably relies too heavily on them. For a young boy who wants nothing more to be a hero, being Quirkless is an insurmountable barrier.
Izuku Midoriya, or Deku, is the series' protagonist. He is introduced as a member of that Quirkless minority. Due to his biology, he faced many hardships in his youth. He was bullied and told his dreams could never come true. However, he was stubbornly determined in the face of adversity and found a way forward. Though he surpassed the unsurpassable obstacle, fans have returned to his backstory to question whether he was really Quirkless to begin with. One chilling theory suggests that he may have had a Quirk, and it was stolen at a young age.
The theory was introduced on Reddit by u/Overhaul_YT. It focuses on how the doctor who diagnosed Deku as Quirkless might have stolen his Quirk and given it to All for One (AFO), Japan's most powerful and feared villain, who enjoys rendering others' Quirks as his own. Though seemingly absurd, the theory surprisingly holds a lot of weight.
Due to the severe injuries AFO sustained from All Might prior to the series' start, a doctor actively oversees his treatment. Viewers catch their first glimpse of the doctor in Season 2, Episode 20's end credit scene. Though he is heavily shrouded in shadows, the glare from a monitor allows the audience to make out a familiar head shape and bushy mustache. Season 5's My Villain Academia arc finally reveals the doctor's full character design. He wears a distinguishable pair of goggles -- similar to the ones worn by Deku's doctor. Though these characters bear a striking physical resemblance, it has not been confirmed if they are the same character.
If Deku's doctor is the same one working for AFO, this might mean he's been actively scouting and stealing Quirks from children. Even when diagnosing Deku, the doctor relays the news in a nonchalant way, which is odd considering that Quirkless people have become increasingly uncommon with each generation. Furthermore, a crucial piece of evidence suggests that the doctor's crimes against children go way beyond merely stealing their Quirks.
In Season 2's Hero Killer arc, a winged Nomu picks Deku out of a crowd. Nomus are created by combining several deceased individuals into one monster. When the series' creator, Kohei Horikoshi, was asked why a Nomu would target Deku specifically, he responded with a teasing drawing of Tsubasa -- a character with a wing Quirk, who had bullied Deku along with Katsuki Bakugo. If Tsubasa was used to create the Nomu, the monster might've retained his ability to recognize Deku as prey. It's also been confirmed that Tsubasa is the grandson of Deku's doctor. Thus, this theory hauntingly implies that the doctor experimented on his own grandson.
If the doctor did steal Deku's Quirk, then an important question remains: What was his original Quirk? Generally, these special abilities are passed down genetically. In Deku's case, he could've inherited a telekinetic Quirk from his mother or a fire-breathing one from his father. A combination of these Quirks might've given him pyrokinesis, but there is no way to know for sure, yet. AFO has not displayed any of these three Quirk possibilities in the series, however, since he's been alive for 9 generations, it's impossible to know how many Quirks are in his possession.
Supposedly, those born Quirkless have 2 joints in their pinky toe. However, medical records can be faked and if the doctor's word is already in question, there is a chance that he could be lying about the toe test as well. That being said, there is too little information to use as evidence here. While MHA: Two Heroes featured another Quirkless character, Melissa Shield, there was no mention of an extra joint. Thus, there was no clear contradiction to Deku's doctor's statement.
Although Deku possibly being born with a Quirk does alter his backstory, it doesn't undermine the struggles he's overcome; he still had to grow up under harsh discrimination simply for being Quirkless. If done right, confirming this theory could add depth to his character and highlight more of AFO's heinous crimes. While this is simply a theory with no confirmation in any canon material, the fan-found evidence is certainly unnerving.