Across nearly every world with any form of powers, there will rise someone who can claim the powers of others.
It's practically a physical law.
The exact nature of how this works varies even more than the nature of the powers of each world.
Kill someone, and gain access to a ghost copy of them.
Copy the ability after beating them in combat.
Copy the ability after being hit by it.
Recreate the powers from a sample of DNA or its equivalent.
Or just directly steal the powers from their original holder.
The only hard rules about these thieves is that the longer they live, the stronger they get.
And they will not die of natural causes.
Of course powers will grow stronger from generation to generation as well, and often thieves have limits that give those from a late enough generation the ability to remove their threat.
On one particularly unfortunate world their thief appeared in the earliest days of their powers.
This thief sought power, stealing from the heroes he fought and innocents alike, even as he gathered his own followers and claimed the notoriety of a Demon King.
Until one day, when the greatest hero the world had ever known, lured him into a trap to confront him.
He made use of the thiefs secret weakness, trapping him alone and vulnerable.
With the help of his companions and the fires that burned around them, the hero banished the thief from the world, into the Darkness Between.
But alas, the thief had one final laugh.
He stole the very power that let the heroes control the fires, willing them to cover the world.
As the surface of the world burned, the thief (a tiny thing really, without his stolen powers) was left trapped between the worlds, waiting to be drawn to another…
It was Monday and Izuku was stepping inside UAs 1-A classroom once more.
He'd remained on edge the entire time since the disastrous first day, terrified of what would happen if he misplaced his special dispensation card.
He had already been stopped by police twice on his way here, having to show the card before he could keep moving.
The second time it nearly hadn't worked, due to one of the cracks happening to lie over the card and following when he tried to make sure the officer could read it.
At least he hadn't had any heroes treating him like a criminal…
As he opened the door to the classroom, he found the rest of the class talking quietly, with the sole exception of Yuga.
For some reason the extravagant boy with the laser Quirk was nowhere to be seen.
Ochako, the gravity negating Quirk holder he'd saved in the entrance exam, was the first to notice him.
"Izuku," she shouted in clear joy, jumping up to run towards him. "You're back!"
She completely ignored the cracks (not to mention normal propriety) to wrap him in an unexpected hug.
Yaoyorozu, of the object creating Quirk, stood up more sedately, smiling at seeing him. "I'm glad to see you really came back. Principal Nezu assured us you would, but we couldn't be sure until we saw you."
"Of course he did, after you convinced everyone to walk out unless he did something," the short kid with purple balls for hair said. "How did you get all the girls so loyal to you so quickly? Teach me your secrets!"
"He made good points," Asui, the frog girl, explained, flicking the boy with her tongue in clear admonishment. "After that, I wouldn't have trusted Aizawa as a teacher. And it wasn't everyone, Bakugou and Yuga didn't join the protest."
"Thank you, all of you," Izuku managed, choked up by the display of… he didn't know exactly. Friendship? Or was that too weak a word? Whatever it was, it was something he'd never felt before.
"All of us except Bakugou, right," Sero, the boy with tape dispenser elbows, responded, grinning in a manner to suggest he enjoyed the idea of winding the explosive blond up. "He left before we even discussed what we were doing."
"Actually," Yaoyorozu started carefully, drawing Bakugous immediate attention.
"Don't you dare," he shouted, to no avail.
"Bakugou had already left to speak with the principal himself."
"How did you even know that," Bakugou demanded, barely holding himself back from using his Quirk.
"The principal mentioned it when I met with him," Yaoyorozu responded calmly.
"I guess you really consider me your rival," Izuku commented quietly, smiling at how included the class was making him feel.
Bakugou huffed, turning away from Yaoyorozu and dropping his Quirk usage.
"Of course I do. As if anyone else could hope to match me."
It was a statement that earned looks from most of the class, given how many of them had scored higher in the test given by Aizawa.
(The results of the test had been compiled, despite its disastrous conclusion, and been distributed by Nezu for his own reasons.)
"Alright little listeners," a familiarly overpowering voice started from the hall behind Izuku, who quickly turned to see Present Mic with a particularly tired looking Eraserhead following behind him. "Good to see you all getting along so well! I'm your new homeroom teacher! Can I get a 'Yeeeaaaah'?!"
The class responded with a stunned silence.
"Must I be here for this," Eraserhead mumbled.
"As Nezu said, the class has Quirks that could cause harm with the slightest of accidents, so it is only responsible to have you present for every period," Present Mic reminded his fellow Pro Hero.
"If I'm going to be present for each period, when am I supposed to sleep?"
"At night, like the rest of us!"
Eraserhead sighed deeply.
"I've learned my lesson, I'm not going to erase their Quirks without good reason. Can't you get Nezu to lighten up on my punishment? Surely you only need me for lessons when they're expected to use their Quirks?"
"That goes against Nezus direct orders," Present Mic disagreed, apparently having the time of his life.
"Um, sensei," Iida hesitantly spoke up, interrupting the brewing argument. "Why is Aizawa wearing glasses?"
"They aren't just any glasses, the support department constructed them! They serve to block Midoriya from his sight, so he physically cannot erase his Quirk again! The design is such that it should precisely match his silhouette, so people can't just hide behind him!"
[%]
The rest of the day saw Aizawa forced to follow them from class to class, each teacher using their own approaches to keeping him awake, from Present Mics natural volume to the way Midnight seemed to keep 'accidentally' catching him with her ever present whip.
This wasn't actually the only strange thing about the day.
Izuku had, at first, thought that Yuga was simply running late, but the flamboyant boy never appeared.
And when he finally found the courage to ask about it, Cementoss (who Izuku couldn't help but find the most approachable of the teachers) merely stated that Nezu was dealing with the matter.
Lunch was also unusual for Izuku, with more than just Bakugou wanting to sit with him, such as Ochako, Yaoyorozu and Mineta (until he wound the girls up too much and was banished to another table).
And then their group was approached by a blond from the other class.
"I bet you're so jealous that you missed out on the excitement last week," he mocked.
Izuku and his classmates (other than Bakugou, who simply ignored the 'background character') shared a look between them.
What excitement?
"Don't tell me you didn't hear that a group of reporters broke onto the grounds over rumours of All Might teaching here, setting off the alarms in the process?"
"You had the alarms go off? That must have been terrifying," was Izukus immediate response, which clearly caught the boy off guard. "I've heard about villains attempting to attack schools before, and I definitely wouldn't want to live through something like that."
"Y-yeah? I guess you aren't a proper hero student," the student said in an attempt to recover. "Aren't you ashamed to be showing off your Quirk like this?"
The mood around the table instantly changed from vaguely amused to murderous rage (as Bakugou was suddenly paying attention).
Izuku stood up, placing himself between the boy and his classmates (despite the voice in his head saying that he shouldn't protect people from the consequences of their own actions, and to let Bakugou deal with him).
"This is the result of a bad Quirk interaction," he stated firmly. "My Quirk is currently as inactive as possible."
"We'll see about that," the blond commented, reaching out towards Izuku.
It was as he made contact that they were interrupted again.
A giant fist came down upon him.
"Honestly, what are you doing," a girls voice asked, and Izuku looked up to see it was the bearer of the hand growing Quirk. "We're sorry about whatever Neito did," she continued, speaking to them this time. "He's an idiot. Some of us suspect that he was taught about social interaction wrong, as a joke. It's about the only explanation we have."
She dragged the protesting Neito away with her.
The members of class 1-A (again, excepting Bakugou) shared a look.
"Well, that just happened…"
[%]
The last lesson of the day was Introduction to Heroics, with none other than All Might himself.
"This lesson today was intended to go over what happened in last weeks lesson," All Might started, after the class settled from his classic 'I am here' introduction (with an added 'coming through the door like an ordinary person' that rather disproved itself and caused Aizawa to glare at his fellow hero). "Going over what went well or poorly, and what weaknesses you have to work on. Of course, due to other events that lesson hasn't yet taken place."
"Does this mean we're doing last weeks lesson today," was Ashidos question for All Might.
He shifted uncomfortably.
"After 1-Bs experiences in the lesson, I was convinced we need proper Quirk training lessons before attempt a repeat. Which is a shame, as it was a good way to learn exactly what everyone needs to focus on…"
"It's a major risk until you know the people you're working with," Aizawa drawled out.
"Yes, quite. In any case, the lesson requires splitting into groups of four, so we couldn't do it anyway. Instead we will focus on your Quirks, and their theoretical applications in heroics. Any volunteers to go first?"
They hesitated.
"Maybe you could demonstrate?"
"Excellent idea," All Might agreed easily. "You all know that my Quirk mainly functions as an enhancement type, boosting my speed, strength and durability. This obviously gives me an advantage in direct combat, where I can take and dish out more hits than my opponents, as long as I modulate my strength properly."
"I heard you can change the weather with a single punch. I don't think that should be possible with just raw strength…"
"Ah, that is why I said it mostly functions as an enhancement type. There are other aspects as well, but they aren't relevant for this level. So, my Quirk gives me an advantage in hand to hand fighting, but what are the weaknesses I had to work on?"
"Holding back the right amount," Izuku suggested.
"You don't have any way to fight at a distance."
"Raw strength is useless without technique."
"Quite right, all of you. With strength comes the need to know instinctively how much to put into any move, to train to actually land hits correctly, and a way to get into the fight. Of course, I am fast enough that that isn't a problem, but it still took time to learn how to react at such speeds, and…"
"I think that's enough," Aizawa drawled out, looking like he was barely keeping awake. "We should move on-"
"Right, Aizawas Quirk next," All Might continued, ignoring any protests the former teacher could make. "The basics are that he can negate any non-mutant Quirk with a line of sight restriction. Useful against villains with emitter or transformation Quirks, but only to the degree with which said villain relies on their Quirk. As such Aizawa has a Quirkless adjacent fighting style for takedowns. Any other ideas on how to use the Quirk?"
"If it relies on sight, does that mean blinking disrupts the effects?"
It took Izuku a moment to realise he was the one who asked the question.
Everyone furned to look at Aizawa.
"... Yes," he eventually admitted.
Everyone looked back at Izuku, who shrunk back on himself at the attention.
"What were you thinking of," All Might coaxed him.
"Well," Izuku started slowly, "clearly the main weakness other than mutation Quirks and unexpected interactions would be how long Aizawa can keep his eyes open for. So if it was me, I'd try to train myself to keep my eyes open for as long as possible, and avoid anything that would compromise that."
"Like staying awake too long, rather than getting enough sleep?"
Izuku nodded, getting into his analysis enough to no longer notice the attention on him. "Exactly. For any vision limited Quirk, the main priority has to be getting enough rest that you won't have to close your eyes at an inconvenient moment."
The glare Aizawa was shooting Izukus general location grew stronger.
"Excellent analysis! Now, we can't spend the lesson focused on us teachers, so let us start on you. Volunteers?"
"My Quirk is Explosion, and…"
[%]
With the day finally over, Toshinori found himself sitting across a table from Nezu.
"You've had a lesson with every class you'll be teaching," the principal commented. "Anyone jump out at you as a successor?"
Toshinori hesitated.
Should he mention Sir Nighteyes recommendation?
"Not really," he instead admitted. "Everyone has the potential to be a hero at least… what's happening with Aoyama?"
Nezu took a sip of tea before answering.
"After the way he was the only student in 1-A not to threaten to leave aroused my suspicions, my investigations revealed that his parents were being held hostage in order to convince him to act as a spy. I currently have your old sidekick on the job of rescuing them and determining whether he should remain a student or if I should help him get accepted into another high school."
"I hadn't realised it was something so serious… Do we know anything about the villain responsible?"
Nezu hummed into his tea rather than answer.
"On the subject of your old sidekick, he mentioned that he had found you the perfect successor."
Toshinori sighed.
"I considered him, but looking at his record here, I fear One For All would be more of a curse than a blessing for young Mirio."
"Oh?"
"He started without much control over his Quirk, and has spent the last couple of years training to be skilled enough to use in a real fight. It feels like… if I gave him One For All I would be saying 'good job with all your hard work! Now, let me render it all irrelevant by giving you an entirely different power you'll need to learn to use!'"
He sighed heavily.
"I think… I told Sir Nighteye how easily I took to One For All, and I think he assumes it will be that easy for anyone. When I know that I'm an outlier compared to the other wielders."
Nezu hmmed thoughtfully.
"All of our students have put in work towards being able to use their Quirks in heroics. If you don't want that effort to be irrelevant, why are you looking for a successor here?"
Toshinori hesitated.
"Every year, a limited number of graduates receive Hero licenses," he finally started. "Less than the total size of all the hero school final years put together. Putting forward a successor from outside that system means taking a place away from one of them. And after all the times I've been called in about people with Mutation Quirks, or passively active Quirks, who weren't able to get the security of a Hero license, I just…"
He trailed off.
Nezu nodded in understanding.
"It's one of the costs of the Commission trying to keep control over Quirks," he stated. "Unfortunately it isn't yet possible to move against them and-"
"Move against them? The HPSC? I know they have their problems, but things aren't that bad, are they?"
"Ah, you know how ambiguous language can be sometimes," Nezu covered.
It wasn't something he spoke about, but he had, after reaching an 'agreement' with the HPSC, discovered that they were responsible for the experiments run on him.
It was part of their efforts to learn as much as they could about Quirks in order to control the country.
The investigation that had uncovered this fact had also uncovered evidence of the HPSC working to covertly breach other aspects of their truce, and so Nezu had started preparing to strike against them in turn.
He wasn't about to overtly break the agreement, not until he was ready to counter their immediate response.
He had made the 'agreement' for a reason, after all.
"Have you considered your course of action if you cannot find a worthy successor," Nezu asked rather than let any of his plans about the HPSC show.