Full Infusion

A/N: The turning point in this story will take place at the USJ arc as this is where things will get interesting. Though ultimately the USJ is just a prologue to explain why things REALLY take a turn so stick through this.

Entering the USJ did not feel like entering a training gym, it felt like entering a theme park with a sense of wonder. In every part of the building were different attractions, though they were, of course, meant to simulate different natural disasters rather than for simple amusement it was still a sight to behold. The building was covered in a gigantic dome, which was especially impressive considering the building itself covered what was the size of a football pitch. Around them were the six rescue zones that he read about online, going clockwise they were the ruins', landslide, mountain, conflagration, flood, and downpour zones, with the central plaza in the middle.

Here, Izuku would be learning how to rescue civilians, which was perhaps the most important jobs a hero must do. Even the most combat focused heroes would have to rescue civilians in a fight, and it was no coincidence that All Might's debut wasn't a villain fight, but a rescue mission. It was the core of being a hero, to use one's power to protect the innocent. Today he would be learning the fundamentals to do exactly that.

It was right at this moment that a purple cloud appeared in the central plaza. As the students started wondering to themselves whether it was all part of the training a man wearing a series of hands stepped out of the portal, followed by a huge, blue gorilla-like beast with a bird beak. Then many more villains followed them through, causing a panic as the students tried to leave the building, only to be cut off by the warp villain appeared and separated all of them to the different zones in USJ.

And that was how Izuku found himself falling into the ruins' zone of the USJ. He landed into one of the alleys and rolled a good few feet before he struggled to pick himself up. His ears perked up when he heard a few grumbling nearby, and dragged himself to where it sounded most active. He climbed onto his feet and found that Kirishima had followed through the portal and landed beside him.

"Where are those student brats? They were supposed to come out of that portal!" shouted a voice, presumably a villain nearby. The two students exchanged a glance and listened intently.

"Must have slipped into one of the alleyways, I swear UA make these models according to the real-life scales. They must've spread out, so they can't have gotten far!" Another shouted, to the sounds of agreement by several more voices; it sounded like a lot.

"So, what do you think? Ambush them as they get close? Between the two of us, we can make this work." Kirishima suggested, silently pounding his hardened fist into his palm. Izuku thought about it but shook his head.

"I know you can hold your own, Kirishima, but if I use my quirk my arm will break, and then I'll be useless." Izuku reminded him, just as Aizawa had warned him in his first lesson. Why hadn't he trained more with one for all and removed this problem? "I don't think you can take them all on alone."

"Damn that's a good point," he sighed, "so any ideas?"

"Maybe if we knew each other's quirks well enough, we can formulate some kind of plan." Izuku suggested timidly to which he nodded.

"Well my quirk isn't all that special, it's really just like I said on the bus. I can harden my skin to withstand most attacks, also it can be used to punch much harder. Other than that it's not much." Kirishima explained, "but it's far better than any of these loser's."

"My quirk stockpiles colossal energy and power." Izuku explained, carefully avoiding the true nature of one for all but still keeping the relevant information, "but it's too much energy that I can't control, so I end up breaking my own arms when I use it."

"Can you not turn it down or something?" Kirishima asked but Izuku shook his head.

"I've been trying, but I just can't seem to get a handle on it." Izuku admitted, "so we have to be careful when we use it."

"If only it was more like All Might's-" Izuku tried awfully hard not to laugh, "-then you would have the durability to endure the backlash." Kirishima sighed, unsure. If only he knew.

"All Might can control his power outage, he doesn't have any super durability." Izuku replied, his own experiences with one for all had made that painfully aware. One for all stockpiled power not strength.

"What are you talking about? Of course he does. How else would he survive half the battles that he's been in?" Asked Kirishima, blissfully ignorant of the full technicalities of All Might's quirk.

…

Wait a minute.

He was right!

All Might had survived being thrown through skyscrapers, pummelled by villains with super strength quirks, and even blasted by all kinds of elemental quirks. While Izuku knew that with All Might's injury he wasn't completely invincible, that level of resistance couldn't be achieved with just muscle, it has to be his quirk. And it was a quirk Izuku now shared, so why didn't he share that durability?

Wait, has he ever tried to increase his durability?

"Kirishima you're a genius," he whispered, a smile erupting on his face. This had been the breakthrough he'd been waiting for.

"I am?" He asked, clearly confused but Izuku didn't care. He needed to figure out how to activate one for all's durability, and fortunately there might be someone who could help him right here.

"Can you tell me how your quirk works? How do you activate, it I mean? How does it feel?" He asked, to a bemused Kirishima.

"Well I kinda just feel the muscle in my body become infused with something that makes them stronger. Some kind of power I guess," he said after a long period of thought. It looked as if he'd never been asked this question before.

But it was all Izuku needed, as it caused him to remember something All Might said to him the day they first met.

"You know how guys at the pool are always sucking in and flexing and trying to look buff. I'm like that."

"I'M A GUY AT THE POOL!"

"What?" Kirishima whimpered, but Izuku wasn't paying attention. His mind raced as he processed things. All Might had actually been talking about one for all, he just didn't say it because he hadn't chosen him yet.

"I MEAN I'M A SPONGE!"

"Mido-bro are you alright?" Kirishima asked, concerned, but Izuku didn't care. Up until now he'd used one for all as a power to call on and had concentrated it solely on his attacks. Now however, he infused one for all throughout his body and into his every muscle, bone and cell. The results were instantaneous, he could feel his muscles bulging and he grew ever so slightly taller.

"Woah Mido-bro what did you do?" Kirishima asked, but it was at this moment that a villain found them. As tall as a wardrobe with huge horns, likely some kind of bull quirk.

"HAH FOUND YOU!" He growled, raising two fists to slam them.

"ONE FOR ALL: FULL INFUSION!" Izuku screamed, sending one for all through his arm as he yelled, "SMAAAASH"

The villain was sent flying, his massive strength useless against one for all, he was slammed straight into a wall and collapsed. The other villains who had come to support their ally stared on in shock.

Izuku clenched his fist, it wasn't broken; it wasn't bruised; it didn't even hurt.

"I did it, I did it, I can fight!" Izuku announced, his mind had turned blank at the power Izuku now wielded. Some of the villains were scared now that may have been the strongest fighter for this group.

"Holy crap bro, that was super manly." Kirishima gushed, joining his side with a firm punch of his own to a different villain, "you ready to send these villains back to wherever they crawled out from?"

"Do any of you want to surrender? I'll give you this one chance." Izuku asked, slamming his fist into his palm, and jumping out of his skin when the resulting shockwave caused the ground to crack, "oops sorry, I still can't control this."

Seeing how powerful he was certainly making a few of the villains pause in fear of the green haired kid. There were seven villains left, each with different quirks. If they didn't surrender then there was no choice, he and Kirishima would have to fight their way out.

"Why are you afraid of two kids? Grow a backbone, we're here to kill All Might! We can't cower in front of a few punk kids!" Yelled one of the women, with two arm blades. She swung her arms wildly at them. No form but with such a ferocity it didn't matter. Kirishima jumped in front of him and blocked her attacks with his hardened skin.

"You didn't forget about me did you?" He yelled as the blades bounced off his skin. Izuku tried to help but at that moment, chunks of concrete flew towards him. The first one hit, it should have killed him, but with Full Infusion it didn't. Two more came but Izuku punched through each.

Another villain jumped him, this one with some lizard quirk. He was strong enough to throw Izuku into a building. Izuku, though, jumped off the building and back at him, delivering a kick which sent the villain into the concrete. He had no time to appreciate his victory. When a slab of concrete soared past him. The hairs on his neck stood at this. By reflex, Izuku sent a punch in that same direction, the shockwave sent the villain flying. Four more came at him, each with various quirks, all close range. Izuku didn't think, he charged. One fell from an enhanced kick, another a punch, one landed a hit on Izuku once and Izuku but he managed to block the other one. The last two were slammed against each other, and through it all Izuku remained unharmed by his quirk.

It was around that time that Kirishima managed to dodge a strike from the blade woman, and then kick her to the floor. Then he delivered another punch to keep her unconscious, before turning to see the villains Izuku fought.

"Holy hell Midoriya you took down all those villains by yourself? That's- that's incredible!" He shrieked, gazing at the villains fallen in awe.

Izuku had fallen into a fight or flight instinct in the battle, with the adrenaline falling, he began to realise what he had done. This wasn't a fight, this was a beatdown and Izuku had never been on the delivering end of one before.

"Do you think I went a bit too far?" He asked quietly, none of them looked like they would be getting up in a while.

"Those were dirtbag losers, just forget them." Kirishima replied, "more importantly, where did you learn to fight like that?"

"It wasn't anything impressive. I just figured out how to use my quirk to protect myself as well as attack. It's not as if I was a good fighter, it was just my quirk." Izuku assured him but Kirishima's awe wasn't deterred.

"Wow, that's super humble and super manly bro. You know I thought Tsuyu was crazy before, but now I think she has a point. Your quirk is just like All Might's"

"Geh- no it's not- I mean they're completely different- I assure you, you're just thinking things!" Izuku babbled in a complete panic.

"Dude I'm serious, you should ask him for lessons," If only he knew.

"Now we can rescue our classmates and show those villains what for," Kirishima shouted and it was a suggestion that Izuku wanted to take. But it had a problem.

"If we gather our classmates that warp villain will just separate us again, we'll need to take him down first if we were to have any hope of helping our classmates." Izuku muttered to which Kirishima accepted.

"Good point. Okay we'll show that bastard that they were wrong to ever set foot in UA." Kirishima growled with a smile, to which Izuku nodded, toning back Full Infusion to stop. His muscles had begun to ache slightly, but he wasn't sure what that meant.

"He's likely at the central plaza, it'll be easier for him to coordinate his attack with the other villains that way." Izuku muttered and Kirishima nodded, the two of them made their way to the central plaza. That was the last place Izuku saw Aizawa, but there was no way he could be doing well. He was outnumbered, and his quirk was poorly suited for large scale engagements even with his training. Hopefully the two of them could help him.

Fortunately, the two of them avoided most of the villains who had separated into the different zones, and had arrived at the central plaza. Where Aizawa still stood, tired and bruised, but still fighting. He was charging at the hand villain now, closing in. He dodged his the villain's hand and elbowed him in the stomach, but Aizawa was still caught.

"It was hard to tell when you were jumping around but I found your tell, it's your hair." Came the thin raspy voice of the villain, Izuku redoubled his efforts to get there fast but he was still so far away. Especially when Aizawa's skin started turning grey.

"When it drops it means you've stopped using your quirk, you're having to blink more often, don't push yourself too hard now. You might just fall apart, wouldn't that be a shame." The villains growled in his thin raspy voice. Izuku was still so far away, enough that the villain didn't notice him and since he stood too far to help.

He needed to get there faster, wait- what about a one for all enhanced leap? Like he had done at the entrance exam, with Full Infusion he could do it without breaking his bones. He re-infused his body, ready to spring into action.

But at first it seemed Aizawa didn't need his help, he has managed to worm his way out of the villain's grip. Kicking the hand villain to the ground as he did so and out-manoeuvring three of the villain's mooks, but he was tired and panting.

"That annoying quirk of yours isn't suited for drawn out fights against big groups is it?" The hand villain laughed, "don't you think you're a little out of your element here Eraserhead? You're much better at working stealthily. You're known for surprise attacks, not fighting head to head. But despite knowing that you didn't hesitate to jump into the middle of this fight, to put your students at ease."

It was like Izuku's own analysis, this was something the two of them shared. And Izuku could see that the villain was right, he needed to get in there. But he had to get this jump right, otherwise he would miss and fly past them, especially as more villains swarmed Aizawa. Even as they were taken down, Izuku could see his strength being sapped away.

"And look at you you're still standing! You really are so cool," laughed the hand villain, "oh by the way hero-"

Aizawa turned to face a huge impending villain. As huge as a Gorilla, with bulging black muscles, a monstrous beak and a brain that bulged out of itsown skull with dead eyes like that of a puppet. It was the same one that exited the portal with the villain, yet seemed so much worse.

Izuku did not think. He leapt into the fray.

"-I am not the final boss."

The beast's giant claw struck at his teacher, it would not miss. The claw came down on him in a gigantic swipe, but Izuku flew in there, his arms reaching around his teacher's waist and they both tumbled out of the way to safety.

"What the-?" the villain gasped, as Nomu hit thin air. The two UA members tumbled to the ground at the coast of the lake, Aizawa was just as surprised as the villain.

"Midoriya you shouldn't be here," he gasped, "you should leave this to us and get out of here."

"I'm not leaving you, you are being overwhelmed. And your quirk is useless on that creature." Izuku pointed out, "I can help you now, I learned to use my quirk safely."

"You're still a student and untrained," Aizawa said, "you're just going to get yourself killed."

Izuku took a breath, in truth his hands were shaking like he'd spent a month in the Arctic, tears of fear threatened to drip leak from his eyes, and his skin was laced with a deathly pale colour. But his mouth moved on his own, and his body acted braver than he did.

"You can't fight these villains alone, but I can help you until backup arrives. We'll have a much better chance of surviving if we face these villains together than if we turn our backs to them." Izuku replied "I may not be a hero yet, but one day I will become one… and I'm going to start by helping you."

The hand villain just cackled loudly, "it seems we've found a secret boss."

A/N: First of all Full Infusion is NOT Full Cowling, nor is it an upgraded version of it. Full Cowling acts by turning the 'switch' of one for all on constantly and running it through the body like a circuit. This allows him to control the output of one for all as well granting increased speed. Full infusion is from this fic only and it infuses Izuku's muscles with one for all, granting increased durability. Enough that he can punch at 100% but cannot reduce the use of one for all or utilise his super speed, but he can use a super-fast jump like he did against the zero pointer.

I imagine this to be a similar technique to how Skel-Might becomes All Might but not nearly as drastic as the change. Izuku might later learn Full Cowling but it'll be much later than the canon.

Also expect a lot of things to go different just because I like to experiment with a few things, it'll be different to what happens in canon, but not to the extent where it's totally unrecognisable.