Chapter Text
When Izuku Midoriya wakes up the next morning, it feels like waking up in a dream.
The cave is bright, sunlight gently trickling down through the hole in the ceiling, making the water beyond his feet sparkle.
A gold halo fills his vision, radiant with the light.
Despite sleeping on the cold hard ground (completely naked), Izuku feels as if he's just slept on a bed full of feathers.
Of course, the merman resting on his chest, drawing aimless patterns with a gentle claw, definitely is part of the equation.
Now that it's completely dry, Izuku sees the messy hair in a new light — gently curling locks standing up in wild ways. Unable to resist, he gently brushes his fingers through the blond hair.
The clawed finger abruptly stops its circling movement.
"I—" Izuku opens his mouth to apologize, not wanting the dream to end quite yet, but is cut off.
"Do you want to leave the cave?" Katsuki breathes the words against Izuku's skin; the feel of his lips moving has Izuku's stomach nearly flip.
And then he processes the words.
"What?"
Katsuki doesn't look at him, instead pressing his nose and face deeper into Izuku's stomach before sitting up. Izuku is faced with the clean lines of the merman's back and takes only a second to marvel at the freckles scattered there. They are so incredibly… human, that Izuku's heart aches for an entirely different reason, recalling all he'd learned about the merpeople.
And then Katsuki finally looks at him and it feels as if all the air were being sucked out of the cave.
His skin glows with a healthy flush, and his deep red eyes are unwavering. Calm. Determined. Resolved. His jaw, clenched.
But when he speaks, his words lack the typical bite, "You wanted to see what was beyond the cave, right?"
Izuku's mouth opens and closes. He can't help the feeling that something isn't… right.
But then Katsuki gives him a small smile.
"Yeah," he breathes.
"Well, come on, Sunfish." Katsuki pulls himself across the sand, tumbling into the water. The splash paints the once-dry sand a deep tan.
Not wanting to be abandoned, Izuku scrambles up — makes one quick effort to scan the cave for his pants which have somehow disappeared — before walking over to the edge.
Beneath the water, he can make out the blurry mirage that is Katsuki. Orange and black scales radiant in the sun-lit water.
Izuku gives one quick roll of his shoulder, flex of his fingers. Is amazed, not for the first time, how effortlessly it has healed, and how incredibly lucky he is to have Katsuki.
With that, he takes a step back and cannon balls into the small pool.
The water rushes around him, quickly engulfing his body. It is much colder than he remembers.
He is not cold for long.
Clawed fingers curl around his waist and his floating body comes willingly when Katsuki pulls him close.
Opening his eyes, he can't make out more than a blur of gold, orange, black and the blue of the water — like some sort of abstract painting.
And yet, he knows exactly when the merman smirks at him.
At the very base of the cave is a gaping hole that Izuku had never been able to see from the surface.
Izuku barely has time to wrap his arms around Katsuki's shoulders, before they're off.
Katsuski's tail shifts and turns, a gentle move that catapults them through the hole and out into the open water.
Everything is a blur, and the salt is starting to hurt his eyes, so Izuku just focuses on holding on. He buries his nose into Katsuki's neck.
The merman, in turn, tightens his hold of Izuku.
His lungs have begun to ache, the quickly deplenishing air causing his chest to tighten. But this doesn't worry him. Despite the looming reality that he may very well drown if he doesn't reach the surface in the next 30 seconds, Izuku trusts Katsuki.
Trusts him with everything.
So Izuku just holds on.
Feels the water whip past them like fast moving wind, except softer, the roll of it caressing their bodies as they cut through.
A hand moves to squeeze the back of Izuku's neck, holding the base of his head as if he were something dear. The other arm, wrapped around his waist and keeping him close.
Between the water and Katsuki, Izuku feels well cared for.
It's nice.
Just as Izuku's chest begins to burn, he feels them slow. Feels the water grow warmer and pressure soften, before his head breaks through the surface. He can't help the loud and exaggerated breath he takes. His lungs greedily gathering each inhale and the oxygen they offer, and when Izuku no longer feels short of breath, he finally takes in the view.
In front of them (yet still a short distance away) is a lush, green island. Small but tittering with life. Birds darting in and out of the tropical fauna.
This must have been where Katsuki was coming to get the wood and food he'd brought Izuku.
The merman was truly amazing.
"Wow, Kacchan—" Izuku turns a bright smile toward the merman, planning to praise not only his fast swimming but this isolated island when the words die on his tongue.
The merman is not looking at him.
No, he's looking at something far off to the side.
Izuku follows his gaze, and sees the vast white sails of a ship.
And that's when he realizes some of the tittering he'd taken for wild life is actually people talking. Laughing. Shouting at each other.
With a hunger he hasn't felt in a long time, Izuku squints his eyes and scans the coastline. They're too far away to make out any details, but even from here he recognizes the bright red hair of Eijiro. The lumbering tall form of Tenya. The quiet red-white of Shoto sitting off to the side, watching his friends with, no doubt, a small smile on his face.
Izuku feels a knot begin to form in his throat and without even thinking he goes to swim toward his friends.
The arm around his waist tightens, and it's then that Izuku remembers the merman.
Katsuki watches him with a reserved expression, both angry and sad at the same time, and Izuku knows that he toes a fine line of whether or not the merman will explode on him or not.
Which isn't good.
Not because Izuku minds being yelled at by the merman. No, that would never be the case. He just doesn't want to see Katsuki sad.
So he swallows and says the only word he can think of, "Why?" The word sounds more choked than he intended.
The merman's expression pulls down into a stubborn frown, "You always wanted to leave and go back to them."
"But—" Not like this. He can't refute the merman, but he'd always thought they'd have more time. That somehow, they wouldn't ever have to separate from each other. "Why?" He asks again, feeling his bottom lip start to tremble.
He doesn't want to leave Katsuki.
And then as if the conversation was exhausting him, Katsuki lets out a sigh, "Our time together always had a time limit, Sunfish. Don't be stupid."
Something about the merman's blasé response, his stiff form which still held Izuku tightly, desperately, enraged Izuku. "I'm not being stupid!"
His outburst even surprised him.
Katsuki's eyes grow wide before his jaw clenches and he comes in closer to Izuku, emphasizing their height difference with the prickling of his ears and fins.
"Yes, you are." He grinds out.
"No, I'm not! Why are you trying to get rid of me?"
"Trying? I don't need to try. What did you think was going to happen, Izuku?"
Izuku flinches at the use of his name. Katsuki doesn't seem to notice, or if he does, he uses it as fuel to obliterate the sailor's heart, piece by piece.
"What?" He sneers, "You thought we'd live 'happily every after'? That I'd move to some quiet human town with you? That I'd forget the murder of my people and play domestic housewife?"
"No, I—"
"Did you think we could fucking erase years of hatred after only a few days together? You don't even know me; do you know how many humans I've killed? How I enjoyed watching them thrash under the water. Helpless. Weak."
"I didn't—" Izuku is floundering, a cliff threatening to crumble under the harsh wind and rain of a ruthless storm.
Because that's what Katsuki is. Blind to his rage. Claws digging in to Izuku's skin as sharp teeth formed words with one purpose: to hurt him.
"I have one goal and no human is going to change that: to take down the emperor. That's why we would never work. You're so fucking naive, Izuku. You think everyone has the best intentions when the world is actually full of shit. When I'm—"
"I don't care!" The words rip out of Izuku, and for a second both merman and human are left staring at each other: chests rising and falling with exertion, hands clinging to wet skin, faces set in hardheaded determination.
Neither says anything for a moment, and Izuku feels the anger die out of him as quickly as it came.
"Kacchan," He starts, and waits until the merman looks at him. "I—Well, I like you—"
"Tch."
"No, listen. I— a month ago I didn't know anything about merpeople. I was totally blind to the empire and what they had taught me all my life, but now I know. And I know you." Izuku blazes forward before Katsuki can interrupt him, "I don't care about what you did. About the people you… killed. Okay, well maybe I wish you hadn't done that to be honest, but I get why you did it. And I'm not scared of you.
"I'm not scared of you, and I'm not leaving you." He repeats when the irritated frown on Katsuki's face veers on the edge of disbelief. "You don't have to be alone—"
"I am alone, Izuku." He bites out, "Your flowery speech doesn't change reality."
"But you're not! Cause I'm here, and I want to help you. I can help you. No matter what, I'm your friend, Kacchan."
Skeptical, Katsuki raises a brow and takes in Izuku's expression — clearly trying to read his mind and intentions.
Which, if Izuku thinks about is, is a win in and of itself. He was expecting to argue some more.
"How?" The merman finally asks, and Izuku tightens his grip on those pale, muscular shoulders.
"I'm going to take down the empire."
"Huh?" Katsuki says, but his disgusted expression isn't enough to stop the boiling excitement bubbling within Izuku. Not when a plan has slowly started to formulate in his brain.
"I'll go back to my crew, and explain everything you told me — they won't be able to stand for it once they find out, especially the captain. With them, we'll create a plan to destroy the emperor's fleet, taking down his first line of defense and ensuring he can never hunt down another mermaid. We'll break into the castle and disarm him, turn him into a prisoner and make sure he can never hurt anyone ever again."
"…"
"What do you think?" Izuku says with stars in his eyes.
"You're an idiot, you know that?" Katsuki looks like he doesn't want to believe him, but there's a hope shining in his inhuman eyes that Izuku clings to.
"You've said."
"You can't actually think you will be able to take down the emperor who is immortal."
"If it means keeping you safe, I will."
"I'm not some damsel in distress like your idiotic human fairytales," Katsuki grumbles, seeming to blush the more the angrier he grows.
"I know," Izuku assures, quietly. "You're stronger than anyone I know, Kacchan."
"Fuck," Katsuki says annoyed and under his breath before shoving Izuku (who was clinging to him like an octopus) off. "Fine, go. Get yourself killed. See how much I care."
With those surly words, the merman turns to leave. In his panic, Izuku blurts out, "Wait!" and the other listens. Turning back to face him, waiting. "Let's meet back here in a year. When I'm done… I want to see you again." The words jumble out of him, the worry that Katsuki will disappear forever, far too real.
"Here?" The merman asks.
Izuku nods eagerly, treading water. Katsuki looks at Izuku, then behind him toward the island, and crosses his arms. At first, Izuku thinks the merman has finally found his resolve and will tell him off. Tell him that he's stupid, an idiot, why would he ever want to see him again?
So his heart utterly melts when, instead, Katsuki shoots him that razer sharp smirk and says, "You mean if you don't die?"
"Yeah. If I don't die. Will you be here?" Izuku can't help how hopeful he sounds.
While the truth is, now that he knows about the mermaids, he would feel the need to take action no matter what, there's still a part of him that is doing this exclusively for the creature in front of him. So that Katsuki can be happy, can live without fear, can see a future not driven by revenge but simply becoming the best he can be.
"I guess you'll just have to come back and see," And with one final smile that will very likely sear itself into Izuku's memory forever and haunt his dreams for these next twelve months, Katsuki dives under the water and disappears.
Which leaves Izuku with a plan to enact, and a merman to come back to.
With that, he starts swimming toward the island. Toward his crew. His body aches from disuse but it's nice to stretch his limbs like this.
When he gets close enough to the beach, someone finally spots him. He hears a shouted hey and someone's out there!
It's Momo who recognizes him first. "Is that Izuku?"
"Izuku?!" Ochaco shouts.
"Midoriya??" Shoto stands up from his spot.
"Wait, bro, why are you naked?!"
Taking down an empire is hard business.
Who knew.
When he finally made it back to his crew, they spent a long time fussing over him, explaining how they hadn't given up hope that he somehow had survived (You're a tough cookie, Eijiro had said, slapping his back) and had been circling the area in hopes of finding him.
He'd been plowed with food, medical attention, and (thankfully) clothes. When All Might finally asked how he'd survived, Izuku dove into the long tale of his savior.
Mina had demanded he detail just how beautiful the merman was multiple times, which had caused Izuku to blush furiously, but thankfully no one called it out.
Momo on the other hand was interested in the biology of the merpeople — she probably would have been interested in their mating seasons too, but Izuku held that fact close to his chest.
When he finally told them the cruel truth behind the emperor's immortality, it was met with cusses, gasps, and shouts of outrage.
By the end of the first night, they all agreed they had to do something.
And, thankfully, their captain had a plan.
They spent the next three months sailing across the empire (and on its outskirts) to find allies.
Naomasa Tsukauchi, leader of a small group of ships in the capital. Shota Aizawa, a retired captain of one of the biggest rebellious forces who lived all the way to the east with his husband. Keigo Takami. Tsunagu Hakamada. Rumi Usagiyama.
And more.
They gathered people far and wide, those who had long since been tired by the emperor's rule and oppression.
Unrest stewed in small towns first, then larger ones, then the cities. Royal fleets were taken out by "rogue pirates." Fights and fires were breaking out all across the empire. It was mayhem, but mayhem that made the roots grow weaker and weaker.
It wasn't until nearly twelve months into their work that they finally found an opportunity to storm the capital and take down the emperor. It took their entire force.
A blood bath, the only way to describe it.
When they finally reached the emperor — hidden behind many doors and armed guards — he was in a bed with doctors flurrying around him. It was almost shocking how weak a man with so much power could look.
It was then that Izuku realized he'd never actually seen the emperor before outside of the images in textbooks.
Clearly for good reason.
It was only after the emperor was captured, sentenced to rot in a prison for the rest of his life, that the biggest unveil occurred.
A clueless doctor, unaware of the destruction happening in the main palace, walked out from behind a large painting, his face quickly paling as he unveiled a secret passageway.
The man tried to shut the door and squirrel himself away but Rumi Usagiyama had been quick, ripping the door open and darting down the stairs.
What they found…
Rows and rows of large upright tubes, filled with a green liquid. And floating inside those tubes, unconscious, were the merpeople that had been captured over the past decades. Next to each was a vial filled with an amber liquid that must be the pure form of amvrosia.
The very thing that had healed Izuku all those months ago.
A knot had loosened in his chest when someone screamed out "They're alive! They're just sleeping!"
The doctors were incredibly uncooperative (until Rumi held a knife to their throats), and after that, they spent the next few hours waking up the merpeople. Explaining what had happened. The fall of the empire. The truth that they would be free in the ocean soon.
Izuku nearly started crying when he saw a mermaid with spiky blond hair, deep crimson eyes, and an orange and black tail. The first thing she said when she woke, a panicked worry to her voice was, "My son! Where is my son!"
Izuku was quick to kneel next to her, eyes tearing up at the bruises that covered her chest and throat from where the amvrosia was taken. "Kacchan is okay. They never got him. He's alive. He's alive." He repeated, and when Katsuki's mother began to cry in relief, Izuku's tears finally fell and all he could hope for was that he wasn't lying to her, that Katsuki had managed to stay safe this past year.
That he had trusted Izuku to follow through.
Books would be written and stories told for decades after about the night the empire fell.
About the beautiful song that filled the air as over a hundred mermaids entered the sea for the first time in a long time.
Or about the celebrations that followed after.
The new government that was created. The plan to include a merperson on the council.
But Izuku wasn't there for any of that. He had somewhere else to be.
And now, scarred with the aftermath of war, battered from months of planning, and alone, Izuku sails toward that little island in the south, hoping he's not too late.
It's been exactly one year and two weeks since he made his promise to Katsuki. A promise that they would find each other again, here, after their freedom was won.
The small ship he had commandeered was slow and had taken time to travel across the sea, but Izuku wouldn't have done it any other way. He needed to complete this journey alone.
The island is just as beautiful as he remembers it: lush with greenery and entirely isolated save for the singing birds.
Which would have been nice, if Izuku wasn't expecting a habitant to be there.
His heart felt heavy as he desperately scanned the horizon from the distance. His eyes bouncing over trees and scraggly rocks, as his ship moved closer and closer.
Maybe he'd taken too long. Maybe something had happened to the merman. Or maybe Katsuki had never had any intentions of coming back.
Izuku's fingers ache from the tight grip he has on the sail's rope, his body leaning forward as he scans and scans and—
Stops. He hears a soft ethereal song in a voice that had once come to him in a dream.
His neck snaps to the right, like a dog searching for its master.
There, sitting on a rock off to the side is a flash of gold and orange. Blindly, Izuku adjusts the sails to follow, his eyes adjusting and taking in the sight like a man starved.
The back of a blond head, hair that has grown. Wide, pale shoulders. Scales that crawl up a lithe waist. Brilliantly bright orange, and sparkling black. A long tail that curls around the rock and aimlessly swishes in the water.
He can't help it when he shouts, "Kacchan!" Needing the merman's attention, needing to confirm this isn't some desperate mirage. They're so close now, if Izuku dove into the water, he could be touching Katsuki in just a minute or two.
When red eyes turn and meet his from over that pale shoulder, Izuku feels like he's able to take his first breath of fresh air in months. His lungs expanding with relief.
And then Katsuki turns a little bit more, and all the air is pulled from Izuku's lungs.
Because there, curled in Katsuki's arms, is a small creature.
A creature with a small black and pale white tail, tufts of green hair, and crimson eyes that stare at Izuku with the same intensity as its mother.
Izuku doesn't think twice before he dives into the water and swims the distance to the rock Katsuki occupies. It feels like hours but can't be more than a moment.
When he surfaces the water, it's to look up at the soft face of the merman who has long been occupying his every waking thought. His heart beats so quickly, Izuku is worried it may actually stop, and then Katsuki gives him a small smirk.
"Took you long enough, Sunfish."