My Hero Academia is one of the hottest shonen anime of recent memory, so it's natural that fans are trying to fill in plot holes that the series itself has yet to explore.
That's where these fascinating My Hero Academia fan theories come in. Like Naruto fan theories and Tokyo Ghoul speculations, these musings may offer answers to burning questions. Who is Izuku Midoriya's father? Who is the traitor Present Mic insists is feeding top secret information to the League of Villains? Why does Tomura Shigaraki walk around with a dismembered hand attached to his face? Maybe these theories can shed some light.
Shoto Todoroki has three siblings, but we only know two of them. Fuyumi is a schoolteacher who inherited an ice quirk from her mom, while Natsuo is a college student with unknown abilities. There's no available information about the eldest Todoroki sibling, but some fans have speculated who he might be.
One popular theory states he's actually Dabi, a member of the League of Villains. His age fits, as he's said to be in his early 20s, which could make him slightly older than the 22-year-old Fuyumi. Not only that, but his fire-based quirk could easily have been inherited from Endeavor.
Dabi also strongly agrees with Stain's low opinion of professional heroes. If Dabi were raised by a man who mistreated his wife and children - and practiced selective breeding to acquire a child with powers he wanted - it's no wonder that he feels that way.
While My Hero Academia canon states that 80% of the population has a quirk of some kind, some fans are skeptical of this claim. According to this theory, rather than being quirkless, 20% of the population simply has a quirk that's useless in most situations.
Reddit user /u/dakotathehuman gives a few interesting examples. A person might have been able to breathe in space, but spent their entire life on Earth and never found out. They might be immune to a specific poison that they never ingest. Or maybe their quirk is noticeable, but doesn't register as a quirk - like their bladder is exceptionally strong, or their farts are completely silent.
There's already an example of a seemingly quirkless person having an ability that no one realized he had. All For One's brother appeared not to have a quirk, but it turned out he had the ability to transfer quirks from one person to another. It's not a huge leap to assume that other quirks could be hidden as well.
Dr. Tsubasa is the doctor who diagnosed Izuku as being quirkless, but there may be more to him than meets the eye. According to Redditor /u/Animeking1108, Dr. Tsubasa is also All For One's doctor. This could mean that he is simply providing him with health care, but it could also mean he's involved in something more sinister.
His son is a winged boy who some fans theorize was used to create Nomus. If Dr. Tsubasa was also doing experimental work for All For One, he might have pulled a Shou Tucker (of Fullmetal Alchemist) and used his own son to create a chimeric monster. If so, that's one of the darkest subplots in the whole show.
Nomus are monsters that are an amalgamation of different humans, and therefore they can use a variety of quirks. It's a pretty disturbing concept by itself, but there's a fan theory that makes it even worse. The Winged Nomu specifically targets Izuku, but there's no clear reason why.
According to this theory, one of the former humans this Nomu contained was one of Katsuki's childhood friends. This friend may have briefly gained control of the Nomu's consciousness and targeted Izuku, as that's what Katsuki and his friends did as children.