AN: I started this fic in 2016 as a gift for a friend, who loves mermaid aus and wanted more. But when it didn't fully fit what she wanted, I put it to the side to work on something that ticked more of her boxes.

Now here I am, giving the gift anyway because why tf not?

Midoriya's morning had been too good for it to last all day. He had woken up on his first alarm with enough time to go to the gym and when he had returned, he discovered that his mom was up and had made a nice cooked breakfast for them both.

From there, the bus had been on time and empty enough that he had a seat for the entire journey. The weather for the walk from the bus stop to his workplace was beautiful. And to top it all off, Uraraka, the receptionist he had a monumental crush on, had pressed a coffee into his hands as he'd stepped into the locker room and they had had an actual, real conversation, in which he didn't just squeak at her and turn into a tomato with legs.

He knew when harried Sero charged through the breakroom that his run of good luck was over.

Without hesitating, he followed Sero into the infirmary, pulling his wetsuit on as he went. The centre wasn't open to the public yet, so the only sounds were from staff members moving hurriedly between areas, ensuring that every tank and every exhibit was as it should be and that all of their residents were fed and ready to work.

Evidently, one resident was not ready.

"Dekuuuu!" came the low growl the moment Midoriya stepped into the infirmary. It was punctuated by the dull 'thunk' of a tail against reinforced glass and water being deliberately scooped and thrown over the edge of the tank.

"Good morning, Kacchan," he said to the agitated merman in the tank, quickly taking stock of his condition.

Based on Sero's reaction, it was not as bad as anticipated. Of course, Kacchan had to be swimming erratically around the tiny tank, a never-ending circle of red, orange, gold and black, which was not helpful for his observations but the fact that he was able to do that told Midoriya that it wasn't too serious. All he could catch sight of was a few lacerations to his hands and forearms and all likely to be superficial.

He sighed. "What did you do this time?"

Kacchan surfaced for just long enough to snap, "Fuck off!" and show him his webbed middle finger.

"He really hates you," observed a new voice.

Midoriya jumped. Todoroki Shouto sat behind him at the computer terminal and now looked from that computer to Midoriya, pinning him with heterochromatic eyes.

Midoriya shrugged. "He's all talk."

At this, Kacchan slammed himself into the side of the tank and immediately ricocheted into the other side. Sero materialised instantly from the back room and knocked against the glass, speaking quickly and soothingly.

Kacchan surfaced once again and screamed, "Fuck off!" before flipping over and splashing heavily with his tail.

"He's really riled up," said Midoriya. "What happened?"

Sero sighed, running a head through his hair. When he turned away from Katsuki, who was now angrily circling the bottom of the tank, Midoriya noticed the bags under bloodshot eyes. "The intern put him in Tank B."

Both Midoriya and Todoroki winced. "Oh no," said Midoriya.

"Who did he maim?" said Todoroki.

"Monoma," sighed Sero, rubbing at his eyes.

Todoroki nodded sagely. Kacchan would absolutely fight Monoma and it was likely that it was an altercation that he wouldn't even have had to start.

"Uwabami managed to split them up-"

"-but isn't Uwabami supposed to be-?" Midoriya muttered.

Sero cut him off. "I know. The intern really fucked up. Anyway, after that, Katsuki tried to escape through the filter."

"Then his hands.."

"Monoma barely got a hit in," Sero confirmed.

"You would think he would have learned his lesson," said Todoroki, moving away, though whether the comment was directed towards Monoma for baiting Kacchan or Kacchan himself for taking on another filtration system, nobody would ever know.

"What's the damage?" said Midoriya quietly.

Sero didn't look at him, watching Kacchan slow his constant circling in the tank until he stopped altogether in the furthest corner and rested at the bottom, pointedly turned away from them both. He looked like a sad, fire-coloured puddle, his beautiful fins, which usually fanned out around him like an explosion of colour, stilled and deflated. The two dark green splodges on his side were plainly visible now that he was still and so were the scars that raked his bare back.

Midoriya's heart squeezed in his chest.

"Monoma will be fine. He has a couple of scratches so he'll be sitting out of the shows for a few days," said Sero quietly. "But this is a massive setback for him. And we were hoping to try integrating him again this week too…"

If Kacchan heard, he gave no indication.

Midoriya frowned, running through Kacchan's options in his head. So far, he had proven to be every bit as explosive as his colouring indicated. Nursing him back to health had been difficult – he tolerated only a handful of humans, of which Midoriya counted himself because despite the constant tirade of profanities and the throwing water and baring teeth and just generally being a pest, Kacchan had yet to physically attack him.

The other merfolk they had tried to socialise him with though had not been so lucky. He was thoroughly hated by tank B and after the first failed attempt at integrating him into their group, he had found himself back in the infirmary tank with a variety of gashes and they had been forced to start the process of introducing him to supportive individuals all over again.

He had hated them all.

They were quickly running out of other merfolk to put him with and were loathe to give him a new tank neighbour he didn't like for very long because of the way he postured at the glass and made the others uncomfortable.

If they could only get him to behave himself around one other mermaid then that could be his ticket to rehabilitation.

But as it was, they were stuck and he was stuck with them. They couldn't release him into any of their exhibits for the public (which was a shame because Kacchan really was a sight to behold) and they couldn't give him to any other trustworthy owner because he simply wasn't ready to cooperate with anybody new.

They had been going to try bringing Kaminari into the tank next door to Kacchan, but any possibility of cooperation from their resident problem child had just evaporated and Sero would now need to lay all the groundwork all over again.

"Midoriya, you're muttering," said Todoroki, appearing at his other elbow.

Midoriya flinched. He had been pinning a focused stare to Kacchan's back and so had been entirely unaware of what was going on around him. Sero, in fact, had left without his noticing – hopefully to take a well-earned break.

"Do you think we should feed Kacchan now?" Midoriya mused aloud.

Todoroki reached around him to grab something from a tray. "It's probably best to leave him as he is while he's quiet."

"I guess."

With a light shake of his head, Midoriya moved towards the whiteboard to take stock of what needed to be done today. The infirmary, thankfully, was empty but for Kacchan in this room and Monoma and Tetsutetsu the other, none of whom required much by way of medical care.

Kendou would be fine with Monoma and Tetsutetsu on her own, leaving only Kacchan and the tank A merfolk as his primary concern. It appeared as though Todoroki had already seen to feeding them all and there was still half an hour to go before they would need to be released into their morning exhibits.

"Did you-" he began to ask Todoroki, only to be interrupted as the door burst open and an exhausted Aizawa strode through with Sero and Iida at his heels.

"But he's –" Sero was saying.

"There is nothing I can do," said Aizawa, dismissively, gesturing for Todoroki and Midoriya to gather. Kendou entered through the storeroom, ushered by Kayama.

"All I'm saying is – can't we delay, just for a few days and put the new patient in the other room?"

"We cannot," said Aizawa with an air of finality.

"But my patient is-"

At this Aizawa turned his head, silencing Sero with a look which he held for a long moment. Nobody in the room dared to breathe. Then he turned back with an exhausted sigh and said, in usual tone, "Listen up because I will only say this once. We have a new patient on the way from the Shiketsu Centre. I'm sure you're all ware of the unlicensed trading facility that was recently raided. Since Shiketsu's medical facilities are full, they are sending the surplus elsewhere. Director Nedzu has signed us up to take one. His notes should be with us very soon if they are not already but as of now, all we know is that the merman we are receiving is stable but will need to be housed in a recovery tank and rehabilitated. Questions?"

"And I'm saying," said Sero, "that you can't put a patient like that next to Katsuki. You've seen how aggressive he is. An aggressive encounter will only distress them both."

Aizawa sighed. "Sero, there is nothing I can do. He needs a tank and we have one. Once space is cleared in the other room then we'll be able to separate them, but in the meantime, we cannot keep a merman sedated in shallow water for five days until a different tank becomes free. I'm sorry, Sero, I am, but you will need to make do with what we have."

As it turned out, Kacchan spent most of the day curled in on himself in the corner of his tank, until Midoriya decided to bite the bullet and try to rouse him while Sero was having a very belated lunch. As the first person Kacchan had tolerated physical contact from, Sero had been the main person in charge of him ever since. Between their other duties, he'd been trying to figure out how to introduce some sort of barrier to Kacchan's tank so the he wouldn't be able to see and antagonise their new patient. Feeding Kacchan was the least Midoriya could do to make his friend's day a little easier.

He took two filleted fish up onto the platform between the two recovery tanks in the room. Though much of Kacchan's history had been missing when he had first arrived, it hadn't taken them long to realise that before he had come to be owned by the creep they had rescued him from, he had been very well taken care of, spoiled in fact.

As a result, though he would eat raw fish, Kacchan would only eat them if they had been prepared like they were being served to a person. He would also, it turned out, swipe things out of people's hands if they were close enough.

"Kacchan!" he called, sitting on the platform and sticking his hand in the water to make it ripple. If the merman didn't hear, he would feel it.

And sure enough, as Midoriya continued to call him, the merman slowly uncurled and turned to stare at him at him with piercing red eyes.

Midoriya held out the food for him to see.

Kacchan glared then turned his back and resumed his previous position.

That was weird. Slightly put out, Midoriya sat for a little while longer with the fish beside him, just in case Kacchan changed his mind. Perhaps he was just still in a mood and wouldn't eat because it was Midoriya offering him it and not Sero? He had done that in the past but Midoriya thought they were over that now.

Kacchan tolerated all of the staff who interacted with him now, even if he didn't like them. It was one of the reasons they had decided to start trying to integrate him into the community of his own species. He was grumpy and had his own roundabout way of doing things, but he wasn't a monster. He wasn't dangerous to them.

If not that though, what could it be? Was Kacchan sick? Had he been hurt more than they had realised? Had it brought back unpleasant memories for him?

"Midoriya," said Todoroki, passing by with a tray covered in instruments for cleaning. "You're muttering again."

"Something's wrong with Kacchan," he replied.

Todoroki remarked dryly, "A lot of things are wrong with Katsuki."

"He isn't eating and he's barely moved all day," he added.

Todoroki sighed. "We'll take a look at him when Sero comes back. In the meantime, maybe you should take a look at the notes Shiketsu sent us."

"Yeah," said Midoriya. He gave Kacchan one last chance to take the food, giving the water another ripple.

This time, the merman didn't even spare him a glance.

Admitting a new resident was always both exhilarating and terrifying. For Midoriya, it wavered closer to the terrifying part of the scale with occasional moments of awe. And yet, it was also always fairly anticlimactic.

In order to minimise stress from moving between their transport tank and the recovery tank, the new patient was usually sedated, and as such, the jobs of Midoriya, Sero and Todoroki right now were mostly centred around anxiously waiting for their new patient to wake up and hoping that they did so as calmly as possible.

"He's got a rare pair of chompers," said Kendou from somewhere behind them.

Midoriya jumped having not heard her come in. She wasn't wrong. Whether the merman had been bred that way or surgically altered, he didn't know – there were no records to say – but the open mouth of the sleeping creature revealed a mouth full of pointed teeth of the kind that a child would draw on a cartoon shark.

Midoriya took a step forwards, his nose milimeters from the glass, but it didn't afford him a much better view. Aside from the teeth, the merman's build was fairly standard. His tail was grey with crimson stripes that seemed to be fake. His arms were littered with bruises and his hands seemed to have taken the worst of the damage, with several claw-like fingernails missing from his fingertips. His injuries could stem from either combat or – like Kacchan – an attempted escape from his tank during the raid.

That was certainly a point to investigate.

"Midoriya," said Todoroki, placing a hand on his shoulder.

"We'll see," said Sero, answering a question Midoriya hadn't heard, his eyes shifting between the new resident and Kacchan in the tank beside him.

So far, Kacchan hadn't stirred, even as the new resident had been lowered into the tank. His eyes were closed in sleep. He was breathing. They had checked the temperature in his tank and found nothing out of the ordinary. It was concerning but it was probably for the best that he wasn't conscious yet – it would give the new resident time to come around and acclimatise.

"What dosage did they give him?" Kendou asked.

Todoroki muttered the reply as the four of them continued to monitor the merman.

"I would say that gives us at least half an hour, wouldn't you?" she said, her tone a gentle reminder that they all could probably be doing something else.

Gentle noises of agreement settled around them and without further comment, they all drifted away. Kendou was the first of them to leave the room, stepping into the chamber that mirrored it next door. Her workstation was set up there, as Todoroki's was mostly in this room. She left both doors open, though all that could be heard was the mocking drawl of Monoma and a swift, decisive hushing from Kendou, before all lapsed into purposeful silence.

Sero was next to leave, heading left past the newcomer's tank, to the storeroom. Their stock didn't need organising; Todoroki and Aizawa were both sticklers for order when it came to medicine and equipment. But Sero's role usually took him out towards Tank A and the merfolk who interacted with the guests, and Midoriya could understand that none of them really wanted to go too far until their new patient has settled in.

"Todoroki?" said Midorya, appearing at his friend's elbow.

Todoroki, who had gone straight to his computer, made a vague noise of acknowledgement.

"Do you need anything?" Midoriya asked.

The next task on his agenda would be doing a general check of their residents and assessing how well they handled the general public that day. Then drawing up rotas for which residents would be where the next day. But that would take him away from their medical facilities.

Todoroki looked for a moment like he was going to say no, but as Midoriya fidgeted from foot to foot, he paused. His piercing eyes stared into Midoriya's for just a moment too long, unreadable despite the pair of them having been friends for years. Then two files were pulled from a somewhat haphazard pile and placed in his arms.

"If you could pick one and have a go at deciphering the handwriting in the areas I've marked, that would be useful."

Almost as an afterthought, a block of post it notes was dropped on top of the folders, and Todoroki turned back to his screen without further preamble.

Midorya grabbed a pen from Todoroki's desk and settled himself in a corner where he was mostly out of the way but could see both Kacchan and their new patient clearly. At the moment, neither was moving, beyond the gentle ripple of Kacchan's fins as he breathed. Not for the first time, he thought that the blonde merman would look beautiful in one of the larger tanks. He pictured Kacchan in the tank with the tunnel where the guests could walk through, his colours swirling around him as he looped around showing off.

Reluctantly, he tore his eyes from the merfolk, flipping over the cover on the first file. It was a case file, incomplete, an uncoordinated assortment of everything the recovery team had been able to quickly gather during the raid and everything the medical team on site had noticed before transporting the patient.

The last one of these Midorya had seen had been Kacchan's. It had been thicker than this. The medical professional on scene had been Yagi Toshinori, Midoriya's personal hero. He'd spent hours marvelling over his idol's big, bold handwriting and detailed observations. Kacchan's original file had been full of notes – Izuku's added to margins in green pen and then stuck to pages on yellow post it notes. Finally, they had been rewtitten on paper and added to the file, which was mostly under Sero's care now, though Midoriya still perused it from time to time.

This one came accompanied by a cursory note from someone called 'Hawks', who had apparently linked this particular merman's journey to three separate criminal owners. He would be sending a subordinate to the centre to investigate once their new patient had settled.

That was new.

The rest of it wasn't. Reading the file, a familiar cold hand seized at Midoriya's insides. His throat tightened and his eyes prickled. They had found the merman in a small, dark room with four tanks just barely large enough to house one fully grown mermaid. They had been crammed two to a tank, with the exception of this one, who had been by himself. There was speculation over whether his tank mate had been the merman found wrapped in tarpaulin around the back, abandoned mid-transportation. Of the five merfolk left in that room, two had been found dead.

Midoriya sniffed but didn't cry. Todoroki spared him a questioning glance as he looked up at the poor creature in the tank next to Kacchan's, then turned back to his computer, mouth set in a grim line. Every injury on the merman's striped body took a new and awful significance. Midoriya's vision misted.

Kacchan finally moved.

The movement was so sudden that Midoriya dropped the newcomer's file and jumped to his feet out of instinct.

"Kacchan?"

The merman drifted upwards, fins still oddly flattened around him, until his face emerged from the surface of the water.

Midoriya approached carefully. "Are you looking for your lunch?"

Kacchan did not acknowledge him. He floated there, nose and mouth just barely above the water. His chest heaved as he drew in gasps of air. His eyes were open but half-lidded and glassy underneath. He looked like a drowned man, moments from giving in to temptation to slip below the waves.

"Kacchan!" Midoriya tapped against the glass, turning back to Todoroki. "Todo-"

His friend was already on his feet. "I know."

Midoriya was up and on to the platform between the recovery tanks before further thought. He reached one hand out towards Kacchan, palm facing upwards. The back of his hand dipped into the water, creating small ripples in the water. He shouldn't. He knew he shouldn't. The rules dictated that he had to be in full apparatus if he was going to touch a distressed mermaid. The rules stated that no appendages should ever be dangled over the edge where they could possibly be grabbed and pulled.

But Kacchan needed his help now. And the merman was just slightly too far away to reach. So Midoriya called to him softly, keeping up a constant stream of calming speech as Sero and Todoroki grabbed gear below.

Finally, Kacchan's dulled red eyes turned to him, his hand with its tattered frills reaching out towards Midoriya's. Midoriya kept his hand open as Kacchan's shaking hand closed around it. The temptation to pull Kacchan closer surged through him, his hand tightening its grip reflexively.

The merman in the water winced and hissed. The semi-conscious look in his eyes seemed to clear, replaced by something that looked like panic. His grip around Midoriya's hand shifted from tentative and shaky to hard, decisive. He tugged as though expecting to be pulled up and out of the water.

Midoriya stumbled forwards with a muffled curse, just managing to hook his elbow around the railing to avoid falling in. A thousand scenarios flashed through his mind.

He'd done it now. This was why there were rules. Kacchan was going to pull under and the press would catch wind of it and Kacchan would be blamed and then he'd have to go and-

"Kacchan!" Midoriya bit out, pain from his arm bringing tears to his eyes. "Stop! Please let go!"

"Midoriya move!" Sero cried.

All Midoriya saw was the widening of Kacchan's eyes and his other hand coming up to grab at him before something heavy hit him in the back. His arm came free of the railings with a horrific crunch.

"Bakugou!" shouted a new voice.

And Midoriya fell face first into the water.

AN: Thanks for reading!

Next chapter will contain some Kirishima pov and backstory.

As always, comments and kudos bring life to these old writing bones.