Gamer71796 - The reveal is here :P
MonsterKing - :D
Naruto30 - TTB's Endgame will, indeed, be the final battle for this fic.
The Northern DireWolf - Aye.
Ricardo Valencia - Aye :P
TheGreatBubbaJ - Few more chapters.
Detrametal - She doesn't kill, she prefers to intimidate (and she knows a few people that DO kill, just ask Shie Hassaikai).
(***)
For as much as a surprise that was, Midoriya Inko didn't murder Izuku for making Aiko participate in the plan to murder Overhaul. But she certainly did pull him by his ear a little while telling the utterly frozen Izuku that if he ever does that again, she is going to show him exactly why his stepfather decided to not fight her over the custody of their children.
Right before hugging him closely and telling him how happy she is that they both survived and that everything ended well.
She also announced to her husband that he's getting a Very Serious Talk (with some additional ear pulling) about allowing all of that to happen while he was supposed to be in charge of the family during her brief trip away.
Once Aiko wakes up, because judging from how soundly she was sleeping, utterly oblivious to the events around her, she had to be really exhausted.
All Might decided to be brave about it. Eraserhead looked like he was hoping that he'd get to see it happen.
(***)
They found Endeavor less than an hour later. Badly beaten up but alive. Air Cannon (reinforced several times) sent him flying and had him hit the ground with a lot of kinetic force, even despite him using his fireblast to lose some of the momentum.
The impact was still strong enough to give him a concussion, two broken ribs, and knock him unconscious until after the battle was over.
He was moved into the hospital, checked out and - once it was obvious that he needed no emergency treatment - quickly patched up by Recovery Girl. He also received an update on the outcome of the battle, after which he promptly departed to look for his son.
He ran into him literally outside of the room where Recovery Girl applied her quirk to him. Shoto Todoroki, clearly, was either waiting for him or actively looking for him. While looking… honestly, bad.
Very emotionless. To the point where it was unnatural. Which, considering the fact that Shoto Todoroki was naturally rather emotionless, spoke volumes about the intensity of 'very'.
"... do you want to talk about it?" Endeavor is fairly certain that he isn't the right person to make a pep talk, but… he can't continue to look down on himself forever. He needs to at least try.
"Yes." Shoto replies, his face unmoving.
Endeavor gestures towards a nearby couch. Shoto goes there, his movements almost mechanical.
Damn, this is going to be awkward as hell, isn't it?
"I heard what happened." Endeavor says. "And I'm sorry." That takes his son off-guard.
"For what?" Shoto asks.
"For failing you as both pro-hero and a father once again." Endeavor replies. "Forcing you to do what was supposed to be my job. Like in Hosu."
"It wasn't your fault." Shoto surprises him. Endeavor deeply expected chastisement, then again… it's his fault, isn't it? Shoto not acting as a normal person would.
"I'm the Number One Hero, Shoto." Endeavor replies. "It's my job to stop people like Eclipse and Overhaul. Yet both of them had to be stopped by you and Midoriya. Say what you want, but I did fail. In Hosu it all played out more or less well, here… "
The words refuse to leave his mouth. This time his failure made his son into a killer. It hurts twice as much, even regardless of the monster and an existential threat to Japan that Overhaul was, when you take what happened to Touya into account.
"How do you feel?" He ends up asking instead after the awkward silence grows too awkward. "About the…"
He feels like it should be Rei who should ask that question. But she isn't yet here. Inko arrived already, but that's mostly because she was still en route to Todoroki's residence when the news went out. In the meantime, Enji will have to do.
"I don't know." Shoto replies, his yes on the wall in front of them. "I think… that I'm happy about it. But I also think that I'm not supposed to be happy about it."
Ah yes. Kick Endeavor some more, remind him of just how much of a failure he was. The fact that his children's mental stability seemed to be reversely proportional to how much time he was spending with them is telling.
"... are you happy that you stopped him, happy that you killed someone, or happy that you and your friends managed to get out of this alive and unharmed?" Endeavor asks. "Only one of these options is truly bad. One is only slightly worrying. One is perfectly understandable."
Shoto clearly didn't expect that. He spends a few seconds thinking about it.
" I think that it's partially the first option and… mostly the third option." He eventually settles up on an answer. "But I'm not one hundred percent sure about it."
Phew. Looks like there is a chance of salvaging this one particular fuck-up of his. He isn't going to let it go to waste.
"Do you want to talk about this?" Endeavor asks. Shoto, after a longer while of thinking about this, nods.
He should be out there, doing politics, trying to clean-up the mess that he really doesn't even want to imagine. So many people dead. So many Top Heroes dead. The Paranormal Liberation Front might have suffered a massive blow, but…
But if there is one thing he learned from All Might then it was the fact that he can't be a hero to the country if he can't be a hero to the people in front of him… and he can't be a hero to people in front of him if he can't be a hero to his own family. So he focuses on the last one for now, while letting Burnin handle the rest.
(***)
"If I knew that this could happen…" Nedzu announces towards Power Loader, the one and only person other than the rat to be present in Nedzu's office on the UA. "... I'd postpone the destruction of the HPSC."
That, Power Loader decides, is the best summary of how incredibly fucked they all were.
"Sir Nighteye was planned to become the head of the committee reviewing ethical conduct among the heroes and investigating potential cases of bad apples among them." Nedzu continues, before sighing painfully. "And with so many Top Heroes dead… we can only hope that we've dealt a blow major enough to the Paranormal Liberation Front for our own casualties to not mat… matter less."
Ah yes, there it was. The sociopathic side of Nedzu. A notable signal of just how distraught he was over all his calculations collapsing on him. Not even due to him making any mistake in particular, just due to some outside factors that he simply couldn't foresee earlier.
Power Loader sighs. Nedzu, of course, picks it up. But decides not to elaborate.
"On the other hand, that confirms our suspicions about some mysterious mastermind hiding behind the Paranormal Liberation Front." Nedzu continues on. Sometimes he enjoys thinking aloud. Typically with someone to listen to him. Typically, it's Power Loader. "I suspect him to be the 'Re-Destro' mentioned by the Nemesis before Overhaul overtook him. But… for some reason I can't get the read on him."
Which, honestly, wasn't THAT much of a surprise. Power Loader realizes that Nedzu doesn't have a lot of intel to work on. They barely have the villain name of the guy.
"Why is that?" He decides to ask. Because in moments like that, he knows that it's his job to ask questions like that.
"Because his handiwork is an unexpected combination of brilliance and stupidity, making me suspect that he is either a genius with some serious mental instability or fanatical beliefs in something…" Nedzu replies. "... or we're not seeing the whole picture."
"... please elaborate?" Power Loader decides to ask. "Because I don't think I'm on the same page."
"It's simple." Nedzu replies. Knowing him, this phrase means that it's anything but simple. "Kamino Ward was, and I don't use that term for no reason, a tactical and strategic masterpiece. Majority of Top Heroes were practically led to a killing ground, and Re-Destro planned it so expertly that they didn't realize that until after the battle ended. I'm simply in awe of that, and honestly, I believe that even All for One would be impressed. The only reason why the hero assault party wasn't utterly annihilated was something that he simply couldn't foresee. But… why did he repeat the whole scenario again?"
"... wait, he succeeded, right?" Power Loader breaks the awkward silence after a few seconds. "So why does it…"
"Because the situation changed." Nedzu cuts in. "Doing it in Kamino Ward made perfect sense, because of the sheer number of supervillains left available to the Paranormal Liberation Front. If they managed to eliminate most of Top Heroes while suffering minimal casualties, something completely possible, nay, guaranteed considering the sheer scope of their plan and the number of prepared redundancy plans, they would deal a near-mortal blow to the Hero Society, which would be left hanging on the HPSC and its paragons."
"... sounds like something that the HPSC president would enjoy." Power Loader mumbles to himself. "
"Actually, I'm not fully discrediting the option that she was either an unwilling pawn or a willing collaborator of that 'Re-Destro'." Nedzu decides to surprise him. "She'd certainly benefit greatly from eliminating the Top Heroes roster, especially if she responded to that by deploying Paragons as their replacement. I find it unlikely, however. First of all, she tended to restrict quirk-usage further, as that reinforced the value of heroes, and through them, the HPSC. If Re-Destro's villain name is any indication, our mysterious mastermind seeks to free quirks rather than restrict them further."
"What if Re-Destro is merely a figurehead for some badly misfired HPSC president's plot?" Power Loader decides to point out. Nedzu thinks it over for a moment.
"Actually, that's a fair idea." The rat admits. "It'd also suggest that the current… mess was mostly due to Re-Destro executing an old plan once again, due to the original source of it no longer being available and him… or her not knowing what else to do. However, this is unlikely for several reasons. To begin with, our favorite former president is… conservative in her planning. Her own type of brilliant, I acknowledge that much, but she was always more interested in securing her own safety than in achieving some major milestone. For example, she recruited her pet assassins from the Paragons so that if their existence was revealed, she could then show the large number of purely innocent Paragons and point out how that one person that came out was delusional, insane or slandering her at a behest of some villains. After all, there are dozens of outstanding Paragons that don't kill people."
"Makes sense." Power Loader nods. "Any other counterargument?" It sounded like there was some, due to the whole 'to begin with' part.
"As stated, the restriction of quirk usage." Nedzu replies. "I can scarcely imagine someone to any degree inspired by Destro agreeing with that woman. Even if told to pretend to have such views, he had to at least read several books on the subject in order to convincingly pretend to be a Destroist. That carries a major risk of him or her going renegade further down the line - you can hate Destro all you want, but his writing skills were incredible. If it was her plot and figurehead, she would pick an aesthetic hitting slightly closer to home."
"... with all due respect, I really think that Hawks' affair proves that her ability to prevent her most important pawns from going renegade on her was… lacking." Power Loader decides to point out another thing. It's a bit of his job to slightly help… 'ground' Nedzu during his strategic musings like this one.
"Right, I think that I still didn't fully process said affair." Nedzu concedes the point. "Back to the main subject. My theory was that Re-Destro's original plan was to destroy the combined forces of the majority of the independent or UA-aligned Top Heroes in Kamino, wait for the HPSC to replace them and then… well, makes you wonder how much he knew about the HPSC little misadventures, hmmm?"
"Oh." Power Loader realizes it. "If he let the president grab full power and then released some inside intel about her assassinations and pawn-grooming…"
"Yes!" Nedzu chirps in. "I believe that this was an original plan. The best case scenario, from his point of view, has the government and the HPSC coming to blows in what would be de facto a civil war, with the majority of the heroes and the entirety of the police and Self-Defense Force siding with the government. HPSC would inevitably lose, however with independent and UA-aligned Top Heroes absent, majority of the heavy-hitters would be on the HPSC side. This means massive casualties on both sides, and further weakening of the government. Even in the worst case scenario, once again for him, the public image of the government and the heroics would receive a massive blow, allowing PLF to rise up."
"Guess that we've really dodged the bullet thanks to Midoriya." Power Loader says. Nedzu agrees with him with a nod.
"However, as I stated earlier, I don't understand his reasoning for what happened today." Nedzu says. He saw the whole thing because Sir Nighteye was kind enough to forward all data from the command post to Nedzu in real time. "The HPSC is out of the picture, yes. And destroying most of the Top Heroes does leave the pro-heroes dangerously weak. However the casualties will be quickly replaced by either rehabilitated Paragons, this time without the potentially compromising data for him to reveal, or former vigilantes. We all know that people like Crawler, Invincible or O'Clock are practically Top Ten Hero equivalents just waiting to be pressed into service. And so is the former Hero Killer. What's more, it feels counterproductive to the PLF's own good."
"I think that you've lost me now." Power Loader decides to intervene. "How is that counterproductive? They just massacred most of the Top Heroes!"
"They did, indeed." Nedzu nods. "But they suffered casualties while doing so. Casualties so crippling that by all intents and purposes the PLF was left in a shape almost as bad as we were. Think about it: Quirkless Liberation Front was decapitated in Hosu, but Overhaul was a good enough replacement for Eclipse. They approached Kamino with their full strength. But during it, the Creature Rejection Clan lost both Purity and Deathmonger, her closest lieutenant. Now Re-Destro casually threw Beast, Curator and Bearhead into the woodchipper, and then tried to push both Overhaul and Chronostasis into it as well. Add almost losing Glassmaker, who might be quick to fold but is still crucial for the PLF's logistics, and we both know that her survival was a total fluke. And that's after losing multiple supervillains and notable villains, like Muscular, Magne and Moonfish. And the latter two were discarded merely to quell Overhaul's suspicions."
Oh. Now Power Loader understands.
"He didn't decapitate only the heroes." He says while Nedzu nods in agreement. "He decapitated the PLF as well."
"Yes!" Nedzu decides to add a vocal agreement as well. "And that's where I feel like I'm missing something. I don't know what Re-Destro's end goal is, I don't know what resources he has available to him. I, quite literally, only know that he exists, that he is called Re-Destro and appears to be the brains behind the Paranormal Liberation Front. I have next to nothing. It is… honestly, I didn't feel as 'hyped', I believe it's the teenager's word for that, about something in years!"
Oh great, the Evil Rat God is back.
"Actually… I'm starting to think that Midoriya's mysterious hacker might be connected to the Re-Destro." Nedzu then immediately defies all expectations. "Re-Destro and Doctor Garaki clearly weren't interested in Overhaul succeeding with his studies. The hacker in question, in the meantime, warned both All Might and our own security that there's an on-going PLF infiltration attempt, thus denying Overhaul any chances of obtaining Eri. But… something, once again, doesn't add up."
"What exactly?" Power Loader asks when Nedzu falls into his thoughts again. "Because for me, it honestly makes perfect sense."
"The mall incident." Nedzu replies. "I don't understand why Re-Destro would be interested in Midoriya. The only potential explanation would be if he knew about Izuku's talents as a mastermind and was interested in cultivating him for some reason. However, considering his or her skills as a hacker, they'd have no problem whatsoever with hacking Midoriya's computer. In fact, it'd be a perfectly logical thing for them to do, in order to monitor someone potentially dangerous. If they did that, they would discover Midoriya contacting Tenko Shimura and persuading both Dabi and Spinner to defect. This means that if the hacker was actually connected to Re-Destro, a villainous mastermind of his stature would undoubtedly manage to counter Midoriya's plan. Or at the very least run some damage control on it."
"... so either that Re-Destro guy underestimated Midoriya and didn't put him under proper surveillance, resulting in Kamino Ward… or what?" Power Loader asks. He is on the verge of getting a headache over all that 5D planning.
"... or there is someone else out there." Nedzu replies. "'Either close to Re-Destro but for some reason playing against him, or another person or group entirely. One whose motives, nature and goals I'm entirely incapable of deducing."
Well, if that ain't ominous.
"I'll start by investigating the Hearts and Minds Party." Nedzu says after a few more seconds. "I suspect that Re-Destro is either connected to it or at least a sympathizer. There is a chance that he is a former disgruntled member who saw the party as not going far enough, or something akin to that. There is also a chance that the HMP is somehow connected to the Paranormal Liberation Front through him."
"Makes perfect sense." Power Loader says. "Just don't tell All Might about it or you might get a mutiny on board."
"I'm acutely aware." Nedzu nods slightly. "In the meantime, I really need to phone the Prime MInister. It's time to run some damage control of our own."
(***)
"I know that this… is probably not the right moment for that." All Might says to Endeavor. The Flame Hero finished consoling and advising his son (actually, it seemed to have gone surprisingly well, even Rei seemed slightly surprised). "But I need you for a moment."
Endeavor was about to leave the hospital with his wife. This was… unexpected.
"What exactly is it about, All Might?" He asks. "I'm a bit b…" His wife tugs his sleeve a bit, before giving him The Look. "How can I help you, Yagi?" He quickly reiterates his earlier words towards All Might, except without the too hostile glare.
"... there is one part of the Overhaul's final battle that I believe you'll benefit from having explained it to you." All Might replies. "Namely, the issue of my daughter's quirk."
"I…" He glances at his wife quickly. She nods. "... Alright. Lead the way." Perhaps they'll finally get some answers as to what happened to Touya. Because he isn't buying the official explanation that his son provided him with.
He didn't 'get his flames under control' the last time. And how repetitive his answers to the queries were, it really sounded like someone told him what to say. And he did visit Aiko Midoriya before it happened.
(***)
In the end, the group to hear the explanations was composed of Eraserhead (who really wasn't sure if he wanted to know the answer), Endeavor and Uraraka Ochaco. The last one, as Midoriya quickly announced, merely getting an advance warning because Izuku wanted to tell the truth to all of his classmates afterwards.
Naturally, she is sitting right next to her boyfriend, holding his hand. Izuku… clearly wasn't doing well after the battle. Eraserhead isn't surprised. She was here for emotional support and that was honestly slightly heartwarming, even to him.
Rei Todoroki decided that her husband needs to learn how to operate unsupervised. And that she wasn't exactly interested in knowing any potential state secrets. All Might and Izuku were naturally present. Together with no longer sleeping Aiko that still occupied All Might's lap.
"So…" Eraserhead decides to break the ice once the silence grows a bit too unnerving. "... is she from a different dimension or not? Asking for Shinsou."
Midoriya actually chuckles at that.
"Not quite, uhm…" He looks at his father questioningly. All Might nods. "This will be a bit of a surprise to you, Endeavor, but before we start, I have to say that she and Asa are actually biological children of All for One."
"Oh." Endeavor finds himself staring at the kid with barely concealed shock. "Does she…"
"No." All Might cuts in. "She doesn't have his quirk. What she got instead is… complicated. In fact, we still aren't sure how to fully classify her quirk. Izuku thinks that it's either a mutation or a complex mutation with accompanying emitter quirk, while I'm leaning more towards an emitter quirk with a significant mutant adaptation. It's also a reason why we aren't settled-up on its name. We don't know if it's just a single quirk or two, and if it's the latter, which part belongs to which quirk."
"She… doesn't seem particularly mutated." Eraserhead comments. Aiko gives him a cold stare. "Unless the combat tentacles are hidden on the inside." This time Izuku stares at him as well. "Asking for Shinsou."
"Right." All Might doesn't seem to have bought that at all. "If I were to summarize her quirk in the simplest words possible, it would be 'she can hear quirks'."
"What?" Endeavor clearly didn't get it. Then again, neither did Eraserhead.
"Use your quirk on Endeavor for a moment." Izuku asks. Eraserhead obeys and, for a second, Endeavor's flame beard vanishes. "Aiko."
Eraserhead lets out a shocked gasp when Aiko's eyes turn red and her hair raises just as he feels his own quirk leaving him. Endeavor is much more reserved, he probably saw a number of copycat quirks during his career, but…
"The most powerful copy quirk in history." All Might announces. "And that's putting it lightly. She can copy quirk merely by repeating the 'sound' they make. We call it 'sounds', because honestly, that's the closest equivalent in human language that she found for it."
Hello eldritch themes, my old friend, I've come to talk with you again.
"You mean that she can… 'learn' quirks?" Endeavor jumped to the conclusion much faster than Eraserhead. "Merely by seeing them being used?" When All Might and Izuku nod in unison, Endeavor's eyes open wide. "That's…"
"...incredibly powerful." All Might finishes for him. "Eventually. The problem is that the sounds of stronger quirks are very complicated to replicate. And the smallest mistake can be potentially fatal. Imagine your own quirk to be a song. She can replicate Hell's Spider if she sees it and manages to memorize the song in question, but all she has to do is miss a note or two and it'll have a completely different trajectory, possibly injuring herself or someone in the vicinity. And if the missing note is the one responsible for her own fire resistance…"
That's… a very easy to recognize scenario. Especially for Endeavor. Painfully so, even.
"Dad actually mentioned another problem." Izuku cuts in. "She can't copy Endeavor's entire quirk, only the applications of it that she got to see. Your every super move is a completely different music for her to memorize. She also can't copy mutant quirks, for obvious reasons. And she also lacks the ability to manifest the biological adaptations, like Uraraka's fingerpads."
"So if she copied my quirk…" Uraraka cuts in. "... it wouldn't work?"
"More like, the output would be severely reduced." Izuku replies. "But even to achieve that she would probably have to patch it slightly, for example by borrowing notes for 'five finger points quirk working without fingerpads' from Tenko. Because yes, she can also, to a degree, remix the quirk-sounds she hears into completely new 'songs'. But it's dangerous."
"So she can practically create new quirks for herself." Eraserhead says. The words… Honestly, it's madness from his point of view. Something that puts everything that he knows about quirks upside down. Endeavor looks like he forgot how to speak. "Although it's time consuming, she needs to memorize the result perfectly and experimenting with quirks that are too powerful carries the risk of lethal accidents?"
"Yep!" Midoriya seems overjoyed about the subject, at least. Then again, it's a quirk. "She actually patched up her own intelligence song from several intelligence quirks she got to witness in the past. She is… well, she is using it most of the time, even now."
"I was tired of being a dummy and not understanding people around me." Aiko finally decides to speak. "Especially adults. Being a child is boring."
"... but while 'sounds' are merely an approximation, she still needs her mouth unobstructed to make it work." Midoriya adds. "Like Inasa needing full control of his hands for his quirk to work. It's the main reason why she hates dentists so much." Aiko actually shivers in the background.
"They are bad." She then announces. "I'm powerless and a dummy when visiting them. I don't like it."
"At least it works like a good motivation for brushing your teeth regularly." All Might says while patting her head. She looks pleased by it. "Right?" She lets out a sound of agreement. Probably.
"... what about Muscular?" Eraserhead thinks that it doesn't make sense. "And Overhaul?"
"Well, she can also… errr… interfere with active quirks in her line of sight." Midoriya replies promptly. "She needs to see them because in a way, she 'hears' the sounds with her eyes, as we said it's super complicated. She altered the functionality of Muscular's quirk, causing it to work completely randomly and… it turns out that having your entire muscular system change shape randomly is rather terrifying."
"Who would have known." Aiko deadpans. Against herself, Uraraka lets out a quiet 'pffft'.
"She did the same to Chisaki, effectively sealing all of his quirks aside from Overhaul and All for One, both of which were at this point Class-Six quirks." Izuku continues. "She can't do anything to them. She also, as stated, can't do anything to mutant quirk holders. She can effectively seal Endeavor's quirk, rendering him powerless… aside from, you know, his physical strength… damn I'm rambling again." He takes a deep breath. "Basically speaking she beats most emitter and transformation quirks easily, for as long as their user is within her line of sight, but is utterly powerless against mutant quirks. Singing more complicated songs also tires her greatly, since she is basically casting her 'magic' from her own stamina. When she grows up, she'll be able to easily defeat Endeavor, but will hopelessly lose against first year hero trainees with mutation quirks. Like Tenya or Tsuyu. So it's a bit of a rock, paper scissors situation. And will still be a bad fit for a prolonged engagement. Besides, she can only keep a single song active at once, so if someone memorizes her moveset, she'll be soundly defeated."
Eraserhead realizes that this means that Aiko battled Muscular while her intelligence song was offline. That was… honestly rather terrifying on its own. Speaking of terrifying…
"And… all that scary acting?" Eraserhead decides to ask. Because, honestly, he had no idea why that part happened.
"Uhm, she wants to be a hero but also have a… rather… villainous aesthetic." Izuku has the decency to look elsewhere while providing Eraserhead with that particular answer. "Like Gang Orca. She wants to be a 'dark magical girl hero', and… well… being slightly spooky is a big part of that. So she decided to start training her acting skills early."
"Are you telling me…" Stay calm, Aizawa. Stay calm. "... that I spend months being afraid, being literally afraid, of a seven years old chuunibyou pretending to be a magical girl?"
"So it worked." Aiko takes that moment to announce, while looking up at Eraserhead. "Cool."
Eraserhead is SO fucking done with her and her entire family. Especially as it's rather obvious that All Might is trying to NOT laugh in the background.
"And I have one final question." Endeavor seems to have managed to (somehow) process the revelation. "What did she do to Touya?" Izuku and All Might both give him a questioning stare. Aiko looks so genuinely innocent that it's suspicious. "Because a day after visiting her he suddenly showed me that he can now use his quirk without hurting himself."
"Aiko." All Might asks while looking down on his adoptive daughter.
"I wanted him to be my minion." She announced. "And his quirk was hurting him. So I fixed that."
Endeavor and Eraserhead have a strange feeling when looking at All Might at that very moment. It's like seeing a PTSD flashback from the outside for a moment.
"Aiko, you know that you were supposed to keep it hidden." All Might eventually settles on saying that. "Or bad people might find out about it and…"
"So I was supposed to let him keep hurting himself for my own gain?" Aiko asks, clearly surprised by the suggestion. Endeavor manages to suppress a chuckle at seeing All Might driven into the corner by his own daughter.
"... we'll have that talk later AND with your mother." All Might surrenders conditionally. "Basically speaking, when she really pushes her quirk to the limit, she can… regulate the sounds of the quirks of other people. She needs prolonged physical contact, we talk about close to an hour of it for smallest changes even, and is never without a price to pay. Judging from what we just heard, she probably traded Touya's excess firepower for fire resistance. So his quirk will never reach the power level that it would normally do, but…"
That explains why his flame declined from blue to white, Endeavor realizes. It was still stronger than his own, but…
"This is also why I'm not all that confused about Dark Shadow worshiping her." Midoriya adds. "Because honestly, from its point of view, she practically is a god. With enough quirks to get inspired by, she would probably manage to create a song… or just alter him in a way that would allow it to exist separately from Tokoyami's body. Even after his death."
"Stop giving her ideas." Eraserhead announces. Neither he nor Endeavor missed the sudden glint in her eyes.
(***)
Time to slightly re-summarize it in the more compact form, not restricted to the confines of a talk.
Quirks are generally composed of two main elements. You have mutations AND a bit of a bullshit reality warping like producing fire out of nowhere [both of those to a degree coinciding with each other]. Aiko's quirk is a form of a mutation quirk that makes her capable of perceiving the 'reality warping' element of quirks differently, with 'sounds/music' being the closest comparison in human language she managed to find.
This means a lot, as she is capable to - if she practises it properly - 'repeat' the songs that she heard, making her capable of practically 'learning' quirks. She is as a result playing a completely different game than everyone else, even than copycat quirks users like Monoma. She is also capable of remixing the notes from the songs she heard into completely new songs, de facto allowing her to 'create' new quirks for herself to use. Think mixing Aoyama's Navel Laser and Tooru's light refraction to make a laser quirk that bounces around the room etc.
She can also, to a degree, alter the songs in her vicinity, but it's rather complicated and tires her greatly [think stuff like Eraserhead's erasure, except she can also redirect attacks etc.]. Once trained enough she can even alter quirks permanently, a big reason why Dark Shadow perceives her as a de facto closest thing to a deity that sapient quirks can have. Both cases operate in a way as if she... 'making correct sound/different sound' at a sound in question until it starts changing itself. She can't, however, make precise guided changes, so what she did to Muscular (random changes completely) and Overhaul (random changes that ended up blowing him and making his Hyper-Regen quirks grow wonky) is all she can do.
She could, TECHNICALLY, do a guided change but she'd have to dissect your quirk entirely, see it used dozens if not hundreds of times and so on.
It sounds overpowered, and pretty much is, but it has a LONG list of downsides. To begin with, she doesn't use quirks instinctively like most people are. Using her powers is tantamount to having to sing a complicated song, in the middle of combat [now that I think of it, she is pretty much a Bard huh]. Slightest misshap and her song backfires or fizzles away. She is practically useless in melee too, because her songs need up to a few seconds to start manifesting once she starts singing [once again, she doesn't use her mouth and normal people don't hear those songs], meaning that in melee she could be easily disarmed by a quirkless guy with proper combat training. Once she grows really trained [like in ETS], she is practically speaking a walking artillery battery that makes Shoto Todoroki outclassed in sheer firepower.
Speaking of, her Extinction Ray from Exiting the Stage that she uses to annihilate the HPSC' Fast Response Unit is probably a combination of some matter annihilation quirk and a mechanism of a quirk that worked like a straight ray, plus several minor quirks for aesthethics (like the glowing 'circle' that the attacked is fired from). She was probably experimenting to make it work for months, ngl. Stuff like walking on the ceiling are much easier since she was probably just copying what she saw earlier (some sort of reverse gravity quirk to be used on yourself) so she wasn't experimenting with creation of a new thing, merely repeating the song for practise (and establishment of her chosen aesthethic).
She can't do shit to mutation quirks. Even at her strongest, she'd probably lose to Miruko as a result - she can't twist her quirk against her AND she is fast and deadly in melee. Hell, Tenya Iida would have a fair shot at her with some good planning, and yes, that includes to a large degree even the ETS Aiko. Another weakness is that the nature of her self-made combat songs is that they are rather 'stagnant' in that she has to be very specific in her song composition - she could copy Endeavor's super moves but she would have much less ability to alter them at the spot. In short - she has a stiff moveset to be learned and countered like a Dark Souls boss.
For example the Extinction Ray only fires in a straight line from point A (the glowing circle in front of her), so it's actually rather easy to evade if you know it is coming.
Worst of all, she has serious issues copying quirks that rely on physical adaptations. Uraraka has her fingerpads - Aiko can't copy fingerpads as they are a form of mutation, so even if she manages to copy Uraraka's quirk song and make it operate off her fingers WITHOUT fingerpads, the quirk output will be severely decreased.