So about the “Aoyama is quirkless” theory...
[Spoiler Warning: This theory contains spoilers for manga chapters that have not yet appeared in the anime. All of the manga spoilers will be below the cut.]
[ID: young Aoyama in flashback from S3E18, asking “Why am I different from everyone else?” There are tears in his eyes and he is clutching at his stomach.]
I don’t think I’ve ever seen a comprehensive post I could link back to, but I’ve seen a few different people float the idea that Aoyama is secretly quirkless and that his “quirk” is really just contained within his belt.
I have a related, but slightly different theory.
I think Aoyama got his quirk from All For One.
Let’s start with the evidence that he is/was quirkless.
First, the flashback from “RUSH!”. Aoyama has consistently, constantly made a point about the fact that he “stands out,” that he’s “special.” In present day, he likes being “different from everyone else” – by which he means, his quirk is very flashy, and the thing he does best is “stand out.”
The Avatar has returned! [ID: Aoyama from S3E18 blasting his navel laser into the sky while Iida watches]
Clearly, the flashback line “Why am I different?” line is meant as a contrast - that “standing out” is something Aoyama has grown into. This entire scene is a significant moment of character growth for him, as he is already proud to stand out, but here he finds a way to actually make it useful and even self-sacrificing. It remains, though, to understand what was different about him as a child to make him so upset. The manga has offered an explanation that I will discuss below, but for the moment I would like to mark that Aoyama’s quirk, by and large, is not that special. He’s flashy, yes, but he is not the most powerful, the most dangerous, or the strangest, even just within Class 1-A. Asking why he’s different from everyone in a world where kids can be born with eight arms and extendable teeth and bird heads seems extreme.
It would, however, make perfect sense as a question if he was quirkless.
Aoyama is the only character we have never to date seen without his support item. He is always and constantly wearing the belt, even when every other character has shed their support items.
[ID: Aoyama from S2E8, sitting on the sidelines of the sports festival, smiling and pointing at his belt. The subtitles say, “He submitted a petition.”]
[ID: one of the heats in the pool races of S3E1. Aoyama is wearing his belt as he uses his navel laser to fly over the pool, racing Todoroki and Sero]
[ID: Aoyama in S3E7, wearing his belt and using his navel laser behind a bush]
The sports festival is the most telling, because we know that Aoyama had to apply for and obtain special permission in advance to be allowed to wear his belt - in other words, Aoyama goes out of his way to ensure he will always be able to have his belt. He’s also had it longer than anyone else has had support items, since he was a child:
[ID: young Aoyama in flashback from S3E18, with an open box and torn off gift wrap beside him, wearing the belt and looking down at it happily]
While there have been some passing references to the fact that civilians can acquire support items if necessary to manage their quirks, it is far from the norm. Aoyama’s condition had to be exceptional to get that belt.
So far, all of this is decent evidence that Aoyama may be secretly quirkless, but then what explains the stomachaches?
[ID: Aoyama from the quirk assessment test in S1E5 with the subtitle “If I shoot for more than a second, I get a stomachache.”]
Theoretically, yes, the stomachache could be caused by however he used the belt, but I think it’s more likely that he does have a quirk, just one that is relatively weak and requires a great deal of support to control. With that in mind, let’s talk about Chapter 168. Manga spoilers start here.