An inevitable truth about series with large followings is that fan theories will abound. This is especially true for My Hero Academia, which has spawned theories ranging from certainly plausible to potentially story-breaking if true. One controversial theory claims that Shota Aizawa, beloved homeroom teacher of Class 1-A and active Pro Hero known as Eraserhead, has been an impostor ever since the USJ incident.
The USJ attack was the first time Class 1-A was brought face to face with actual villains and truly understood the dangers of the heroic path they'd chosen. When Izuku and his classmates were isolated from the rest of UA, the League of Villains invaded their training ground using Kurogiri's Warp Gate Quirk. They quickly filled the area with scores of low-level villains and one of the most powerful Nomu shown in MHA so far.
When the villains cornered Class 1-A, the only Pro Heroes present were Eraserhead and Thirteen. The latter, being a more rescue-oriented hero, stayed with the kids to offer close quarters protection while Eraserhead leaped into battle. As he fought, his years of practice and experience showed in the way he defeated foe after foe despite being grossly outnumbered. It was only when the Nomu entered the fray that the Pro Hero was finally bested. It brutalized him with sheer power, and it is amid this confusion that the theory claims the swap happened.
For this theory to be plausible, Eraserhead's double would have to possess an imitation Quirk far superior to Himiko Toga's, or be a perfect clone of the UA teacher while staying loyal to All For One. Eraserhead being an impostor would explain several mysteries, such as how he was able to recover from his severe Nomu-inflicted injuries in such a short time. It would also explain how the UA traitor got such accurate and timely information on the students' whereabouts, and how All For One knows so much about Izuku Midoriya and All Might's relationship. It's a believable enough theory, but unlikely to be true based on later events in My Hero Academia.
Firstly, Eraserhead has proved on multiple occasions that he truly cares for his students. It'd be natural for an impostor to imitate their original's behavior, but he'd be taking it too far in this case. Even when all signs pointed to Katsuki Bakugo descending into villainy, Eraserhead stood up for him on national television, taking full responsibility for behavior he didn't cultivate or fully understand. On different occasions when Izuku or Ochaco Uraraka needed a pick-me-up after their ever-growing responsibilities weighed them down, Aizawa was always ready with an encouraging word or reassurance to boost their confidence.
Also, his Erasure Quirk has long since been coveted by All For One's right-hand man, Dr. Garaki. If Eraserhead had indeed been swapped out with the original being held captive, they would have wasted no time in duplicating and strengthening his Quirk. Seeing as there aren't Nomu running around with the power to erase the Quirks of Pro Heroes, it's unlikely Eraserhead ever fell into the hands of My Hero Academia's mad scientist.
