Chapter Text
The first time Katsuki Bakugo turned into a wolf, he was buying vegetables.
It wasn't exactly the heroic scene one would expect for the origin of such a monumental problem, but there he was: in the middle of the bustling Mustafu market, with a bag of onions in his left hand and a receipt he was trying to put in his pocket with his right, when the world suddenly became enormous.
Or rather, he became shorter. Hairier. And completely, absolutely, fucking covered in ash-blonde fur.
—What the fu—? —he tried to say, but what came out of his throat was a deep growl that sounded alarmingly like a bark.
The lady at the vegetable stall screamed. A kid pointed, excited. And Katsuki Bakugo, the number one hero in the making, Dynamight, the man who had faced disaster-level villains with a fierce smile, felt for the first time in years the irrational urge to hide under something.
Calm down, he told himself, trying to take a deep breath (which was difficult with this new snout). It's just a quirk. Someone attacked you. Identify the threat. Neutralize. Get back to normal.
But when he tried to activate his explosion, all he got was a pathetic sizzle from his front paws that singed a few hairs and nothing else.
—OH MY GOD, A WOLF! —someone shouted.
—No, wait, it's Dynamight! Or is it? —said another.
—Since when is Dynamight a wolf? That doesn't make sense!
Katsuki growled, this time with genuine frustration. Could people stop asking stupid questions and help him find the bastard who did this to him?
That's when he saw it.
About ten meters away, in the middle of the chaos that was beginning to form, there was a rabbit.
A green rabbit.
A green rabbit with darker spots on its ears and eyes that were too big and bright, looking around with a mix of panic and scientific fascination.
Katsuki knew it immediately. He couldn't explain how, but he knew it.
Shit.
The green rabbit — because it couldn't be anyone else, because the universe had a twisted fucking sense of humor — looked directly at him. Its ears perked up. Its whiskers twitched. And then, in a movement that no wild rabbit would ever make, it started hopping towards him in a straight line, dodging feet and shopping bags with an agility that could only come from years of hero training.
Katsuki wanted to yell at it to stop, to stay where it was, that it was dangerous. But all that came out was a growl that, to his horror, sounded much less threatening than intended.
The rabbit — Izuku, his damn brain insisted on calling it Izuku because he knew it was him — reached his front paws and stopped, panting slightly. Its ears fell back in what Katsuki, with a knowledge he definitely didn't have until five minutes ago, identified as submission or fear. But its eyes looked at him with that characteristic intensity, as if it were taking mental notes.
Stupid nerd, Katsuki thought with something that might have been affection if he allowed himself to examine it too closely. Even as a rabbit, he's still a nerd.
—Bakugo! Midoriya!
Kirishima's voice cut through the chaos. His friend burst through the crowd, hair already hardened in hero mode, closely followed by Aizawa-sensei who moved with his characteristic deadly calm.
—What happened? —Kirishima asked, stopping dead at the scene: a blonde wolf and a green rabbit, staring at each other, completely oblivious to the chaos around them.
Aizawa knelt down, observing them both with a tired expression. He was wearing his civilian clothes, which meant he was officially off-duty and therefore, extremely annoyed.
—Tell me it's not who I think it is.
The wolf — Katsuki — growled what could be interpreted as a greeting.
The rabbit — Izuku — sneezed and then nodded its head. Literally nodded.
—They're who you think they are — Kirishima confirmed, and there was something between resignation and an amusement he knew he shouldn't feel.
—Fantastic. —Aizawa ran a hand over his face—. Kirishima, call Hound Dog. We need discreet transport and a place to keep them while... while this lasts. This case hasn't happened for many years, it's a strange quirk, there isn't much information.
—What if they don't turn back into humans? —Kirishima asked quietly.
The wolf rose onto its hind legs with a clumsiness that betrayed its lack of practice, and let out a sound that in any other context would be a roar, but in this one sounded suspiciously like "OF COURSE I'LL TURN BACK INTO A HUMAN, IDIOT!"
—They seem to understand — Aizawa observed, raising an eyebrow—. That's... useful. And worrying. But yes, they will turn back into humans, we just don't know exactly when.
The rabbit nodded again, then pointed with a paw towards Katsuki and then towards itself, making a circular gesture that probably meant "it happened to both of us at the same time."
—Well, at least they keep their intelligence. —Kirishima crouched down to their level—. Hey, guys! Relax, we're going to figure this out, okay?
The rabbit hopped one step closer to Katsuki. The wolf, for his part, lowered his head slightly in a movement that none of the humans present knew how to interpret, but in the canine world was a gesture of protection.
Aizawa noticed it. Aizawa noticed everything.
—Interesting — he murmured—. Bakugo is in protective mode.
—What? —Kirishima squinted—. Seriously? The Bakugo who once said "feelings are for the weak"?
—People change. Or wolves. I don't know. —Aizawa shrugged—. Let's go. Gather their clothes before someone pulls out a phone and this goes viral.
The first 48 hours were logistical chaos with moments of unintentional comedy.
The temporary residence Aizawa had secured was a spacious apartment on the outskirts, owned by a retired heroine who was traveling. It had enough rooms for everyone, easy-to-clean wooden floors (a precaution Aizawa deeply appreciated), and most importantly, privacy.
Katsuki had returned to his human form exactly three hours after the incident, in the middle of the living room, with a sound similar to a deflating balloon and completely naked.
—WHAT THE FU—?!
Kirishima, who was making tea in the kitchen, turned his head just in time to see his friend covering himself with a sofa cushion.
—Hey, bro! Good to have you back!
—DON'T LOOK AT ME, YOU DAMN IDIOT!
—I've already seen you in the locker rooms, it's nothing new.
—IT'S DIFFERENT! BRING ME A DAMN BLANKET!
From the other side of the room, the green rabbit — who was still a rabbit — made a sound that could be described as a stifled laugh. Its ears trembled.
Katsuki glared at it.
—Shut up, nerd. You're going to change back too and the same thing will happen to you.
The rabbit tilted its head, as if saying "I hope so," and then gracefully hopped towards the coffee table, where there was a notepad and a pen. With a dexterity that surprised everyone, it grabbed the pen with its mouth and started writing.
How do you feel? Notice any side effects? Headache? Blurred vision?
Katsuki read the message over Kirishima's shoulder, who had approached curiously.
—I'm fine, damn nerd. Worry about yourself.
Izuku pointed to the pen and then to himself, shrugging. Then he wrote:
I've been like this for 3 hours. I can't change. I've tried concentrating but nothing happens. How did you do it?
Katsuki frowned. The blanket was now tied around his waist like an improvised sarong.
—I don't know. I was thinking about exploding something and it just... happened.
—Interesting. —Aizawa entered at that moment with a cup of coffee—. It seems the trigger might be emotional or related to the quirk. But in Midoriya's case...
Izuku shook his head. He knew what he wanted to say.
—He can't. Without OFA, his quirk is null. —Aizawa took a sip—. That complicates things. Bakugo has an activation pathway that Midoriya doesn't. He could stay like this longer.
The rabbit shrank. Literally shrank, as if trying to take up less space. Its ears fell back.
Katsuki felt something strange in his chest. An uncomfortable sensation he didn't want to examine too closely.
—Hey, Izuku. —His voice came out harsher than he intended—. Don't worry. We're going to figure this out.
Izuku looked up, surprised. Then, slowly, his ears perked up a little again.
Kirishima exchanged a glance with Aizawa. The professional hero raised an eyebrow but said nothing.
Day two brought Izuku's first change.
It happened while Kirishima was making breakfast. Katsuki was on the sofa, still in human form (he had learned to keep clothes nearby after the incident), reviewing information on his phone about possible quirks that could cause transformations.
—Hey, Kirishima, do you think this could be related to that villain from last week? The one who controlled emotions?
—I don't know, bro. That guy was arrested, right?
—Yeah, but he could have left something behind. A damn trap, an imbued object...
The sound of a body falling to the floor interrupted him.
Both turned. In the spot where the green rabbit had been, Izuku Midoriya was now lying, completely naked, with disheveled hair and an expression of total confusion.
—KACCHAN! —was the first thing he said, and then he realized his state—. AH! WHAT—?! WHERE ARE MY CLOTHES?!
Katsuki felt his face heat up. He turned his head so fast he almost hurt his neck.
—DON'T LOOK, IDIOT!
—YOU DON'T LOOK EITHER!
Kirishima, ever so helpful, was already throwing him a blanket.
—Relax, Midoriya! Here you go!
Izuku wrapped himself quickly, cheeks burning. Katsuki risked a glance and confirmed that, yes, the nerd was covered. He took a deep breath.
—How do you feel? —he asked, recovering his composure.
—Strange. —Izuku frowned—. Like... I don't know. Like my body doesn't quite fit. Does that happen to you?
—At first. It goes away.
—Interesting. —Izuku started talking faster, entering analysis mode—. So adaptation is progressive. Did you notice changes in your senses? Hearing? Smell? Because when I was a rabbit I could smell things from meters away, and my peripheral vision was incredible but I couldn't see well up close, which makes sense because rabbits have eyes on the sides of their heads to detect predators, although it also means they have a blind spot right in front—
—Izuku. —Katsuki cut him off, but without the usual edge—. Breathe.
Izuku blinked.
—Ah. Yeah. Sorry.
—Don't apologize. —Katsuki stood up, adjusting his own blanket—. Let's find clothes. We can't stay like this all day.
As they walked towards the bedrooms (Aizawa had had the foresight to buy spare clothes for both), Izuku stumbled over his own feet.
Katsuki caught his arm before he fell.
—Careful.
—Thanks, Kacchan.
Their eyes met. For a moment, the atmosphere charged with something neither of them could name.
—Tch. —Katsuki let go of his arm and kept walking—. Don't thank me. I just don't want you to get hurt before we solve this.
Izuku smiled, that soft smile that made Katsuki want to look away.
—Sure, Kacchan.
Day three, Izuku woke up as a rabbit.
Again.
He had returned to his human form during the night, sleeping peacefully in the room they shared (because it was easier to watch them if they were together, Aizawa had argued, that and they had left one room free just in case). But when Katsuki opened his eyes that morning, the first thing he saw was a green, fluffy lump in the next bed.
—Again? —he murmured, rubbing his eyes.
The rabbit lifted its head. Its ears fell back in what was clearly frustration. It hopped once on the bed, as if trying to force the change, but nothing happened.
—Relax. —Katsuki sat up, not bothering to cover himself much (they were getting used to the post-transformation lack of clothes)—. You managed to change yesterday. It'll happen again.
Izuku shook his head and hopped off the bed. He went to the table where they had left the notebook the night before and started writing:
I don't understand why it's so hard for me. You change almost whenever you want. I can only do it when my body decides.
Katsuki read the message and frowned. It was true. Since that first time, he had managed to change at will — or at least, when he concentrated enough — but Izuku... Izuku seemed trapped in an unpredictable cycle.
—It might be because of OFA — he said, thinking out loud—. Your body is used to dealing with borrowed quirks. Maybe this is different.
Izuku tilted his head, processing. Then he wrote:
Do you think I could stay like this forever?
The question, written in shaky handwriting, hit Katsuki in the chest in a way he didn't expect.
—No. —The word came out firmer than he intended—. You're not going to stay like this. We're going to solve this shit, you hear?
The rabbit looked at him for a long moment. Then, slowly, it nodded.
Katsuki got up and started to dress. The last thing he needed was for Kirishima to walk in and see him having a serious conversation with a rabbit. Although, come to think of it, that had already happened three times in the last two days.
—Come on — he said, finishing buttoning his pants—. Time for breakfast. And today we have another round of tests with Aizawa.
Izuku hopped along behind him, and Katsuki couldn't help but notice how he stayed close to his feet. How, despite being a rabbit, he seemed to seek his protection.
It's instinct, he thought. Rabbits seek shelter with larger animals. Nothing more.
But when they reached the kitchen and Kirishima smiled upon seeing them, saying "Look, the pack!", Katsuki didn't correct him.
He said nothing.
The tests that day were... revealing.
—It seems the quirk affects each of you differently — Aizawa explained, reviewing the notes he had taken during the morning—. Bakugo, you can change at will, but when you're in wolf form, your animal instincts are more pronounced. Midoriya, you have difficulty changing, but when you do, you retain more conscious control.
Katsuki, now in human form, nodded. Beside him on the floor, Izuku was still a rabbit, but his ears moved attentively as he "listened."
—The current theory — Aizawa continued— is that the quirk adapts to each person's personality and biology. Bakugo, your combative nature and your explosive quirk give you an affinity with a predator. Midoriya, your tendency to analyze and your lack of a quirk of your own make you more compatible with prey. It's... terribly logical, actually.
Katsuki growled.
—And what does that mean for him? Is he going to stay like this forever?
—We don't know. —Aizawa was direct—. It could improve. It could get worse. It could be that he needs a stronger trigger to change. Like an intense emotion, or...
—Or?
—Or the presence of his natural predator. Although technically, rabbits have several natural predators being herbivores, but the wolf in certain areas is one of them.
Everyone looked at Katsuki.
The person in question blinked.
—Are you telling me that I'm the damn trigger?
—Possibly. —Aizawa shrugged—. When you're in wolf form, Midoriya becomes more alert, but also more... calm. As if your presence anchors him. And when you change to human, he seems to relax enough for his body to consider it safe to transform.
Izuku, from the floor, started writing furiously in his notebook.
So if Kacchan stays with me when I'm a rabbit, could it help me change faster?
—In theory, yes. —Aizawa read over his shoulder—. But there's also the opposite risk. If Bakugo, in wolf form, were to lose control...
—I'm not going to lose control. —Katsuki cut him off flatly—. I'm a hero. I control my damn instincts.
Aizawa looked at him for a long time.
—Bakugo. They aren't your instincts. They're a wolf's. Your human brain can make decisions, but your wolf body reacts out of survival. Don't underestimate that.
The silence stretched on.
Izuku, sensing the tension, hopped softly and placed himself between them. He looked at Katsuki with those big rabbit eyes, and then wrote:
I trust Kacchan.
Katsuki read the words and felt something break and rebuild at the same time in his chest.
—Tch. —He looked away—. Of course you can, nerd. I'm not going to fail you.
Kirishima, who had been silent the whole time, let out a small sigh.
—You guys are so... —he began.
—Kirishima — Aizawa warned.
—I wasn't going to say anything.
—You lie very badly.
That night, when everyone was asleep, Katsuki couldn't fall asleep.
From his bed, he could see Izuku's small form in the next bed. He was still a rabbit. He had tried to change before sleeping, concentrating with that intensity only he possessed, but nothing had happened.
Now he slept curled up on the pillow, his green ears partially covering his eyes, his small chest rising and falling with each breath.
He looks so fragile, Katsuki thought. And that bothered him. Because Izuku wasn't fragile. Izuku was the damn hero who had faced Shigaraki, who had carried the weight of One For All, who had broken and rebuilt himself over and over again.
But now he was a rabbit. A rabbit that trusted him.
Katsuki remembered Aizawa's words. If Bakugo, in wolf form, were to lose control...
—It's not going to happen — he whispered into the darkness—. I'm not going to let it happen.
The rabbit stirred in its sleep, making a small sound.
Katsuki stayed awake, watching him, until the first rays of sun began to filter through the window.