Harper

I'll try to present my interpretations because that way it's easier to see where I might have gone wrong.

“We never see him copying a mutant-type Quirk in the manga. In other words, he can't copy mutant-type Quirks!”

Well, yes, but Monoma is a minor character. We see hardly anything of him, so it makes sense that we wouldn't see every detail of Copy in action, either. Why would Horikoshi explain something in detail if it serves no narrative purpose? In short, absence doesn't equal confirmation.

“He didn’t copy OFA, so clearly there are Quirks he can’t copy.”

Monoma copied OFA. This is not only explicitly stated in the main manga (chap 217) but also confirmed outside of it (Monoma's Relationship Chart from the *Ultra Analysis Book*). Even if it weren't stated in words, the fact that Monoma had the same visual effects as Deku makes it clear (chap  214), and this is even more noticeable in the anime, given the visible green lightning (ep 99). Yes, Uraraka said Monoma was bluffing about touching and therefore copying Deku (chap 214), but she isn't a reliable source. I mean, she didn't even know he had extended his Copy usage time (chap 215), so why would she be an authority on the matter? There was no way for her to know that Copy could be active without being used efficiently…after all, it’s not as if she had enough contact or closeness with Monoma to know that kind of detail! In fact, Deku realized Uraraka was mistaken even before Monoma explained the situation to him (chap 216).

So why didn't he succeed with OFA? Well, to put it very simply, OFA is the result of two Quirks. Yoichi, the first user, possessed the Quirk Bestowal, meaning he could pass his Quirk on to others. However, since Yoichi initially had nothing to pass on other than the ability to transfer his Quirk itself, there was a mistaken impression that he was Quirkless. Then, using AFO, [his brother] transferred "Power Stock" to him, a Quirk that allowed the user to stockpile power. Because both believed Yoichi was Quirkless prior to this, they didn't immediately realize he now possessed two Quirks, which combined to form OFA (chap 193, chap 407).

When Yoichi died and his blood splattered onto Kudo, he unconsciously transferred the power to Kudo (chap 369, chap 408). This was possible because the transfer relies on only two conditions: that the OFA user is willing to pass it on  and that the transfer occurs via DNA, the latter requirement being met by the blood in this instance and by hair in more recent cases (chap 2, chap 414). Initially, not much changed because Yoichi didn't have much to pass on, but it marked the beginning of it all. Since Kudo already possessed his own Quirk, Gearshift, the OFA accumulation factor stockpiled his Quirk as well. And when he passed OFA on to Bruce Lee, who already had the Quirk Fa Jin, Bruce became capable of using Gearshift, and his own Quirk was stockpiled too. And so, the process continued!

Monoma copied OFA directly from the source, that is, he didn't receive it from someone else. For all intents and purposes, it was as if OFA were "new", similar to how it was with Yoichi. However, OFA on its own isn't useful to its first user because, precisely because they are the first person, there is no accumulated power that has been passed down to them. That is why, even though Monoma copied and activated OFA, nothing happened when he tried to attack Deku: no accumulated power had been transferred to him, so it was as if Monoma were Quirkless. It is similar to how Yoichi himself was mistaken for someone without a Quirk.

This becomes even clearer when he attempts to use Eri’s Rewind (chap 217). We know he copied Eri because Monoma has a horn just like hers. However, Monoma tells Aizawa that he cannot use Rewind because it is a Quirk that requires a specific resource to be stored and, in this case, it is something he lacks. Monoma illustrates this using Fat Gum as an example: he could copy Fat Absorption, but it would be useless because he doesn't have the necessary body fat to utilize it. In other words, the Quirk is there, but it is unusable. If Monoma were fat enough, he could use Fat Gum’s power without difficulty. Monoma refers to these as "Blank" or "Dud" Quirks, that is, Quirks he successfully copies but cannot effectively use. He explicitly tells Deku that the same thing happened when he copied his Quirk. So…it is explicitly emphasized that he did copy OFA.

In short: Monoma copied OFA, but he didn't have the necessary stored-up resource, which made the Quirk practically useless. It’s the same case with Rewind, and the same thing would happen if he tried it with Fat Absorption.

“Deku was worried about Monoma using OFA because of his physique. This proves that Monoma's body doesn't change with the Quirk. Since a mutant-type Quirk would require that kind of alteration, it cannot be copied”.

Personally, I don't think those two sentences have any connection to each other, but since I see people acting as if they do, I will argue as if that were the case.

An important detail, established from the very beginning of the story is that Deku didn't have the ideal body at the start either (chap 1). He literally had to train before the power was passed to him, because All Might feared what would happen to Deku's body if he used OFA without having a reasonably good physique (chap 2). The fact that Monoma doesn't have the ideal body when copying OFA has nothing to do with OFA itself, since the power transfers Vestiges and Quirks, not physical conditioning. And in any case, the Vestiges are linked to Quirks, as noted through the plot involving AFO.

Furthermore, there are other instances in the main manga proving that Copy literally alters the user’s body. Monoma used Bakugo’s Explosion without blowing himself up or suffering any burns, so he clearly copied Bakugo’s resistance to his own Quirk (chap 29). He used Kirishima’s Hardening, which is a body-altering Quirk (chap 29). We see him with a horn after copying Eri’s Rewind, and that horn could only be there because Monoma’s body was capable of growing it (chap 217).

Clearly, Monoma is capable of altering his body, as long as it's something related to the copied Quirk. Bakugou's resistance to explosions is part of his Quirk, altering the body to harden it is a crucial aspect of Kirishima's Quirk (It is not without reason that it is classified as a Transformation type of Quirk), and Eri's Quirk requires her horn for activation. On the other hand, Quirks like Yanagi's Poltergeist or Kodai's Size do not require any physical alteration, so nothing changes about him physically (chap  210).

Furthermore, we literally have an example of what happens when someone possesses a Quirk but lacks the necessary physical or biological prerequisites: Aoyama. He was born Quirkless and received Navel Laser from One For All (chap 336). This made him capable of firing a laser from his navel, but because he acquired only the Quirk itself, without all the original user '’'s essential genetic traits, it harmed him physically (chap 168). On the other hand, we’ve never seen anything of the sort happen with Monoma, so we can only assume that he copies not just the Quirk itself, but also its biological requirements (such as Bakugou’s resistance to his own explosions).

“Mutations are permanent, but the Copy is temporary. It's a contradiction!”

Well, in theory, Bakugou's resistance to explosions is also kind of permanent, even though it isn't a mutation-type Quirk. If he had to consciously activate it for it to work, he would have blown himself up as a child, no? After all, he didn't have any training or knowledge about his own Quirk back then. And yet, Monoma copied it. So, the fact that a trait is permanent for the original user doesn't prevent it from being temporarily acquired via Copy. It is true that Bakugou might have only developed that resistance at age four rather than having it from birth, but that doesn't change the fact that, at least since the age of four, it has been permanent.

“That would require an immense effort from Monoma’s body. In other words, it wouldn't make sense!”

It would require it, sure, but why does that make it impossible? I mean, imagine the changes needed for a Quirk like Mt. Lady’s to work! Her entire body changes size in a way that doesn't harm her! Yet, not only is it possible, but the size change doesn't even take that long. Toga transforms into other people using her Quirk until she is physically identical to them, and depending on the situation, she can even use the target's Quirk…all of which requires even more extensive alteration than "Copy," which simply copies the Quirk itself! 

You could argue that it isn't always worth it because, for example, the effort involved in sprouting wings on your back only to use them for ten minutes seems disproportionate. But it’s just a matter of Monoma weighing whether the headache is worth it. It’s a limitation that affects the duration, not the success of the Copy itself. This still doesn't confirm that mutant-type Quirks can't be copied. 

Furthermore, why are we assuming that Monoma's limit will be 10 minutes for the rest of his life? We’ve already seen that the time limit can be extended through training: in the Sports Festival, Monoma had a five-minute limit (chap 29), but by the Joint Training arc, that limit has increased to ten minutes (chap 215). What prevents Monoma from increasing time even further by training even harder? Eventually, the time limit might become long enough to make undergoing that kind of transformation worthwhile. I mean, the guy became a top 10 pro hero in the future (chap 431), and I think it would be difficult for him to reach that point with the exact same skills he had in his first year, so something must have improved. What prevents time limit from having been one of those things?

“Erasure cannot interact with mutant-type Quirks. So, Copy doesn't interact with mutant-type Quirks either”.

First of all,let's assume that's definitely the case…what does that have to do with Copy? The way one Quirk functions isn't necessarily the same as another. The Quirks of Toga, Monoma, and AFO are more similar to each other than Aizawa's is to any of them, yet they all have quite obvious differences; so why should Copy be prevented from having different limitations than Erasure?

Secondly, I admit I have my doubts about this. It is true that Erasure couldn't handle the Nomu during the USJ Invasion, but Aizawa himself makes it clear that this is because the Nomu's physical strength isn't the result of a Quirk (chap 17). It would be like Aizawa using Erasure on Endeavor and expecting it to make him physically weak, even though his physical strength has nothing to do with his Quirk. Furthermore, among the invaders was a woman with snake-like hair, which appears to be a mutant-type Quirk (for instance, Shiozaki, who also has a hair-based Quirk, is a mutant), and Aizawa had no trouble dealing with her (chap 14). In the anime, it is even clearer that the snakes in her hair stop moving (ep 10). Aizawa also explains in detail how Erasure works. Despite the name "Erasure”, Aizawa doesn't actually erase Quirks! In fact, he merely deactivates the Quirk factor expression. The example used is notably a mutant type, by the way. More specifically, a person with a tail, similar to Ojirou, whom we know to be a mutant. In that example, Erasure doesn't make the tail disappear but prevents it from moving (chap 135). This also aligns with what happened to the aforementioned snake-woman, since Aizawa didn't remove the snakes from her hair, but merely rendered them unusable.

In short: Erasure interacts with Quirk factors, including Mutant-type ones. Aizawa cannot, of course, erase the physical appendage itself, but he can make the factor be inactive, that is, he can inhibit its expression. Let's say he used Erasure on Pony, for example. Her horns would remain, so if she wanted to impale someone with them, she still could. However, she wouldn't be able to use her "Horn" Quirk, that is, she couldn't produce new horns from keratin or manipulate them. Likewise, if he used Erasure on Ojirou, it would be just like the example from Chapter 135... the tail would still be there, but Ojirou couldn't control it.

“Copy works solely through the expression of a Quirk factor, just like Erasure.”

The idea is that if Erasure interacts with the expression of Quirk factors, then Copy does too, and therefore couldn't truly copy Mutant-type Quirks. Following this logic, if Monoma were to copy Ojiro, for example, he would gain the ability to control a tail, but that would be useless since he doesn't actually have one. This makes more sense than "Erasure and Copy don't interact with Mutant Quirks at all”,  yet it still doesn't hold true when it comes to Copy.

Again, we saw Monoma grow a horn when copying Eri. If Copy only replicated the Quirk expression rather than the genetic aspects of the necessary physical structures, he would possess the power to rewind living beings (even if he couldn't actually execute it due to a lack of stored energy) but would lack the horn required for the ability. Yet he does have the horn, so we know his Copy interacts with Quirks in a “deeper” way than Erasure does.

Another detail is that we know for a fact Erasure works this way because Aizawa explained it (chap 135). On the other hand, whenever Monoma's Quirk is explained or exemplified (e.g., chap 29-30, chaps 209–217, chap 344, chap  347, Ultra Analysis Book), there is no mention of anything of the sort. So, it hasn't been confirmed that Monoma's Quirk copies only the expression of the Quirk factor.

And again, why are the limitations of Erasure indisputable evidence of the limitations of all Quirks that interact with other Quirks? Hawks' Fierce Wings is a mutant-type Quirk, and we saw that AFO was able to steal it (chap 385). In other words, we know that AFO is also capable of affecting mutant Quirks on a physical level. That alone contradicts the idea that, just because Erasure only affects the expression of mutant Quirks, all other Quirk-targeting Quirks must operate the same way. It is true that AFO received only the wing stubs, but that is because Hawks's Quirk factor was already damaged after Dabi burned him (chap 267, chap 354), not because AFO's Quirk was incapable of stealing mutations. If Dabi had never damaged Hawks, AFO would have obtained the complete wings.

In any case, this limitation maybe doesn't seem to affect Copy, since Aizawa, like Hawks, had his Quirk Factor damaged (in his case, the eyes) to the point where Erasure was unusable, yet Monoma was still able to copy it as new, rather than copying the damaged version of it like AFO did with Fierce Wings. I say "maybe" because the difference might be that what Aizawa damaged were normal eyes, which Monoma has. Hawks, on the other hand, had mutant appendages damaged. In other words, something AFO himself didn't originally possess and relied on a stolen Quirk to obtain. Perhaps if Monoma copied a damaged mutant-type Quirk, he would face the same problem as AFO.

Other materials

Now, moving away from the arguments, I want to talk about material other than the main manga because, as mentioned in the first point, there isn't much about Monoma there, and I wanted to look at the other material. I would prefer not to have to rely on them, since they aren’t written by Horikoshi and I particularlly consider their canonical status questionable, but in this case, it is necessary.

Firstly, there’s the manga spin-off Team-Up Missions. In chapter 29, Monoma uses Kendou’s Fist  And, well, her Quirk involves increasing the size of her hand! That certainly counts as a bodily alteration! Yet, Monoma used it without any problems. In other words, Monoma is literally capable of altering his own body. He also used Tetsu’s Steel in Chapter 44, which, like Kirishima’s Quirk, is a Transformation type that requires bodily alteration.

Secondly, the School Briefs novel, specifically in sections II.5 and V.5. The Quirks copied by Monoma, in no particular order, are:

Erasure (Aizawa Shouta).

Tape (Sero Hanta).

Softening (Honehuki Juzo).

Half-Cold Half-Hot (Todoroki Shouto).

Here, I want to draw attention to two Quirks: Todoroki's Half-Cold Half-Hot and Sero's Tape.

First, let's start with Todoroki. In the main manga, there are story arcs centering on the theme of physical compatibility, with the Todoroki family's Quirks being examples. And that is relevant here.

Endeavor believed that his Quirk, Hellflame, had a problematic disadvantage because it offered excellent fire resistance but lacked the ability to regulate temperature. Consequently, although he rarely suffered burns, he would overheat if he used his power for too long (chap 188). This fact, combined with his apparent inability to surpass All Might, led him to believe that a Quirk limitation might be holding him back forever; he concluded that, in that case, he should produce a child capable of achieving what he could not (chap 31, chap 165). Seeking a child without this disadvantage, he pursued a "Quirk Marriage", that is, finding a partner whose Quirk, when combined with his own, would theoretically result in a stronger genetic combination. Since Endeavor’s problem was overheating, he wanted a wife with an ice-related power from the Himura family to counterbalance it. In short, the fire gene came from the Todoroki side (Enji) and the ice gene from the Himura side (Rei). Thus, an ideal Todoroki-Himura child would possess the benefits of both (chap  31, chaps 301-302).

This idealized child is Shouto, who indeed needs to use both aspects of his Quirk to achieve balance, as evidenced by his fight against Deku (chaps 38-40). But before him, there were children with genetic combinations that Endeavor considered less than ideal. Fuyumi and Natsuo are easy to understand: they inherited Rei’s Frost from the Himura side of the family, but did not inherit the fire aspect from the Todoroki side. Therefore, they "failed" to possess both aspects that Endeavor wanted.

But Touya/Dabi presents an interesting case. His Quirk, Blueflame, is a combination of genetic factors from both parents, yet it lacks the balance found in Shouto’s Half-Cold Half-Hot. He possessed the ability to generate and manipulate fire like his father, but his flames were more powerful, as evidenced by their blue color (chap 291). However, while he did possess resistance to fire like Endeavor (otherwise, he would have been burning himself constantly, rather than only after prolonged use or abuse of the Quirk. Like, the first time he manifested his Quirk, he would have lost his hand, at the very least), Touya primarily inherited his mother's resistance to ice. Touya’s fire resistance might suffice for ordinary flames, but it is inadequate for the intensity of his own blue fire. Consequently, he burned his own body during prolonged use (chap, 291, chaps 301-302, chap 349; chap 374; Vol. 36 Touya sketch). At a certain point, he also Awakened the ability to use ice (chap 387), which finally gave his ice resistance a practical purpose. He even was able to use Phosphor, the same attack Shouto uses, which combines both ice and fire.

In other words, the main manga makes it clear that, although both Shouto and Touya possess ice and fire powers, their Quirks are not the same:

Half-Hot Half-Cold: a basically ideal balance of both fire and ice, if used correctly. The user can employ ice to prevent overheating and fire to avoid freezing. When Shouto faced difficulties during the Sports Festival, this was because of his strategy rather than an inherent disadvantage of the Quirk itself. He possesses resistance to both fire and ice, though he is not immune. This is primarily evidenced by the scar Shouto bears after Rei, in a desperate attempt to soothe a scalding water burn, applied ice directly to the wound, resulting in a scar. This is further reinforced by his need to defend against Dabi, which makes it clear that Blueflame has the capacity to injure him. It makes sense, given that Endeavor could also be harmed by Dabi despite his own fire resistance. However, it is worth noting that those specific Quirks belonged to Rei and Dabi, not Shouto. In fact, Shouto has never injured himself using his own power. The user can also combine both aspects, as demonstrated by the use of "Phosphor."

Blueflame: it has both genetic factors, but in an unbalanced way. The user's fire resistance is low compared to the power of the fire itself, which is hotter than ordinary fire. If Dabi produced ordinary fire, I imagine it might cause less harm. The user possesses great resistance to ice, a trait not particularly useful for someone who primarily uses fire. Dabi actually had the capacity to use ice, but it manifested late as a desperate measure to stave off death (Awakening). Perhaps if it had manifested earlier, he might have suffered less damage over the course of his life. He could also use Phosphor, which means he was also capable of combining both aspects of the Quirk.

In short, one thing is clear here: the physical resilience of the Todoroki and Himura bodies is part of their Quirks. Dabi’s greater resistance to cold than to fire is part of Blueflame, just as the inability to regulate body temperature more efficiently is part of Endeavor’s Hellflame. Conversely, the fact that Shoto suffers from neither of these disadvantages is part of Half-Hot Half-Cold.

Now, Monoma did copy Shouto. Granted, it was brief and for silly reasons, but he did copy it. He struggled to use it, leading him to rely heavily on the fire side (much like how Shouto tended to favor ice), but that was due to a lack of training (similar to how he tells Aizawa he would need to train to truly use Warp Gate efficiently in the main manga). Even with that initial lack of control, Monoma didn't injure himself while using the fire. Clearly, Monoma's body was altered to possess the same level of resistance as Shouto's. That being the case, I imagine that if Monoma had copied Dabi, he would have faced the same drawback where the "Blueflame" was far too powerful for his body's resistance to handle. In other words, Monoma's body still undergoes changes, but only regarding factors directly tied to the copied Quirk.

Now, Sero's Tape. I don't even need to say much about this one. Suffice it to say that Sero is classified as the Mutant type. His body is physically structured to accommodate his Quirk, and that is a constant state. He can't just make his distinctive elbows disappear at will, they are like that all the time (and if Aizawa used Erasure on him, Sero’s elbows would likely remain the same, though he wouldn't be able to control the tape). Yet, Monoma copied Tape and used it. In other words, he successfully copied and used a Mutant-type Quirk.

However, an interesting point is that Sero's mutation is directly linked to the copied Quirk, Tape. Without the elbows modifications, he wouldn't be able to use it. If Monoma copies Quirk-related factors (such as Shouto's heat resistance and Bakugou's resistance to explosions), it makes sense that he would copy the mutation inherent to Tape. 

Personally, I prefer to specially avoid merchandise, since it tends to focus even more on fanservice than on lore compared to the spin-offs, but there is one specific official merch that I feel answers all the questions. It’s this one:

Monoma is evidently copying Pony's Horns. We know it is indeed Copy because, in the original image, Tetsu and Kendou are also imitating Pony, but merely making a horn gesture rather than actually having them like Monoma and Pony do. Pony is a heteromorph, meaning she has a permanent equine appearance, whereas Monoma does not. And why is that? Because Monoma only copies Pony's Quirk, which is the ability to produce keratin horns and control them. The only physical alteration Monoma requires is the horns, and the equine aspect has nothing to do with the Quirk itself. Thus, he did not copy Pony's equine mutation…not because copying mutations is impossible, but because that specific mutation has no correlation to the copied Quirk. For example, in a hypothetical scenario... if Monoma copied Tokoyami, he would likely have a Dark Shadow but not the crow head, since the latter isn't necessary for the Quirk to function, meaning the mutation aspect wouldn't be copied. On the other hand, if he copied Jirou, he would necessarily have to alter his ears because her Quirk requires them to work, meaning the mutation likely would be copied. I think it makes sense, and although that merch was probably created with cute fan service in mind rather than lore or world-building, I feel like it worked well in this specific case.

BONUS: I don't consider games because they have to be more liberal in their mechanics for the game to be more fun and It also has visual limitations (imagine if they had to create different models to accommodate all the possibilities of Monoma's copies), but I find it funny that one of Monoma's lines to Shiozaki, who has a mutant Quirk, in MHUR is "will my hair get longer too?" and Shiozaki replies "you're on unknown territory". Apparently, the game developers had the same doubts as the anime/manga fans lol.

So…

So, I got the strong impression that Monoma can copy mutant-type Quirks, but the level of the copy is limited. But that’s no big deal, since Copy is limited for other Quirk types as well, whether due to time constraints or training limitations. Like, he won't copy just any mutation, only those directly related to the Quirk copied. But I always see the idea that Monoma can't copy mutant-type Quirks treated as an unquestionable fact, so I'm wondering if I got something  wrong. Or maybe, since it's been years since I read the manga, I just forgot? I don't know. Anyway, I'm posting this, and if anyone knows the answer, I would love to know!