Archive of Our Own ^beta

Kirshima hums happily as he moves through the water, his powerful tail slicing through the heavy underwater currents like butter as he heads for his special place. It’s just a cave, but it's a cave far away from the responsibilities and expectations that his Kingdom sets so heavily on his shoulders. Even an Emperor deserves a little peace and quiet to relax every now and again.

He wonders if more of those beautiful Upper World trinkets have washed into the cave again. Usually there are only two or three a year, but now they’re washing up almost constantly, and he’s eager to see what the next one is. 

The mershark breaks past the surface of the cave with a soft gasp, just to feel the cool, fresh air travel through his gills and into his lungs. Breathing Upper World air is so much stranger than breathing underwater, but he enjoys it just the same, even if he can only breathe it for so long.

It’s late afternoon right now, and the sun has been shining in the cave just long enough to make the rocks feel warm when Kirishima leans against them, his tail flicking gently in the water. Big, red eyes take in the beauty of the sky above him. Sure, the view of his ocean is as gorgeous as always, but something about the way the sky stretches on and on, reaching as far as the eye can see is breathtaking as well.

Almost breathtaking enough to make him miss the strangled gasp behind him.

Almost.

He whips around immediately, red eyes searching the darker part of the cave behind him only to land on the shocked face of an Upper Worlder.

He’s overwhelmingly small, no taller than Kirishima’s forearm, with eyes that sparkle like rubies and blond hair that rivals the afternoon sun itself. He’s standing on the flat surface of the cave behind him and holding what looks like...a fireball?...in the hand that he’s holding above his head illuminate the cave that much more.

He's wearing some kind of...garb? It was a gorgeous emerald green color at the top that slowly blended into a bright orange as it traveled to the bottom, and at his neck and waist rested beautifully crafted golden chains, delicately sparkling in the afternoon light. A good portion of his torso was bare, with only two thin lines of fabric cupping him underneath his chest, and there are slits up both sides of his lower cloth, showing off two slim yet powerful looking legs.

He is still, save for the gentle rise and fall of his chest. Kirishima is fascinated. He's never seen one of the Upper Worlders so close before, and such a wonderfully pretty one, too! High, beautiful cheekbones, small, delicately pointy ears. Full, pink lips...such an exquisite creation.

He reaches out curiously, wondering if the male's hair was as spiky as it looked, only for the man's face to twist with anger as the soothing warmth in his hands turned into dangerously spitting sparks.

"Get the fuck back!"

He pouts sadly, retracting his hand.

"I am doings apologizing."

The man freezes again, blinking up at him with even more shock than before.

"You speak?!"

"Yes! I am learned from teachings of books," Kirishima proclaims with a proud grin.

"Books?"

"Yes! Books are...small! With shapings like square? And are havings many pages-!"

"I know what the fuck a book is,"  the man snaps, but he seems to be calming down in the face of Kirishima's enthusiasm, even though he's still scowling very ferociously. "How do you know what the fuck a book is?"

"Ah! I am havings many books in kingdom! My mother's give! They are teachings me many words from very small years!"

"Kingdom?"

"Yes! I am beings Eijirou Kirishima, Emperor of the Ocean and MerPeople! Very pleased to be makings your introduction!"

The man straightens, blinking up at him quickly. 

"The MerPeople emperor?"

"Yes! All Merfolk and ocean creatures are beings under my command. I am beings like your Upper World Queen and King!"

"You know the king and queen?" The man demands.

Kirishima shrugs, letting himself relax once again against the warmth of the rocks behind him. It won't be much longer before the sun sets and takes the warmth with it, so he has to get what he can.

"Not beings myself personally. King and Queen Bakugou have only visitings the Mer Kingdom once, and I was only small Mer when they did. My mother's say they are good leaders! Strong and fearless! Care about the people!"

The man scoffs suddenly and looks away, glaring at the ground.

"They care about some people…"

Kirishima frowns, his head tilting to the side, but before he can ask, the man is glaring at him.

"Oi. Have you been stealing my stuff from this cave?"

"Stealing my stuff?"

"No, my stuff. The things that were in this cave. They belong to me. I would leave them here so I could be comfortable, but they would be gone when I came back. I came to set a trap, because I thought it was just a regular thief, but now I think it might be you."

Kirishima just blinks.

The Upper Worlder huffs loudly, rolling his eyes.

"Okay. In this cave, did you find a brush?"

"A...brush…?"

"It's a golden stick, with a round top. The back is covered in jewels, and the front has white bristles?"

Kirishima blinks, and then grins eagerly.

"Yes! The sparkle stick! I have! Is very soft!"

"Is very mine ."

"Oh, I am doings apologizing! I am not knowing the sparkle stick has home! What else is beings yours, small Upper Worlder?"

"Oi! I have a name, you giant fuck!"

"But you are not telling me this name, so what am I should be callings you?"

"My name is Ba - Katsuki. My name is Katsuki."

"Katsuki! What a good namings!"

"You're goddamn right it is," he replies proudly, his chest puffing outwards.

He has two round circles atop his torso with two little balls on either side of a small nub that Kirishima eyes curiously. No Mer has them. Maybe it's just an Upper World thing? He kind of wants to touch them…

Kirishima shakes his head vigorously, trying to put such thoughts to the side so that he can focus. 

But it’s really hard.

"S-so, what else is beings yours from the cave, Katsuki?"

The blond sighs and settles down onto the stone ground of the cave, finally letting his fireball extinguish with a puff of smoke. "Fuck it. You can keep it. I don't really need that shit."

"Is it not beings important to you?"

"No. It's just everyday shit," he grumbles back, leaning back on his hands.

Fuck, those little nubs were really making him curious. He can’t stop himself from asking about them a second longer.

“What are beings those?”

“What are what?”

“Those,” he says, using the very tip of his claw to try and poke at his chest.

Katsuki slaps him away before he can get too close, covering the round protrusions with his hands as he blushes something fierce.

“W-what do you mean what are they?! They’re nipples! Everyone has nipples!”

“I do not!”

The blond pauses, his eyes dropping down to the Mer’s chest. It's true. He doesn’t have nipples, just the faint sheen of scales curling around his powerful torso muscles.

“Huh.”

“What are they beings for?”

Katsuki shrugs awkwardly, not moving his hands. “Men don’t use their nipples, really. But females use them to like, feed babies and shit.”

“Babies are beings the small human children, yes?”

“Yes. When females have babies, their breasts make milk for them because they can’t eat regular food, and it comes out of their nipples.”

“What are beings the small balls on them?”

“They’re piercings. My family gets them when we come of age as a symbol of adulthood.”

Kirishima ‘ooh’s and ‘ah’s slowly, his gaze sparkling with interest.

“And you are tellings me that male nipples are not beings able to make milk? But then why are the males being havers of nipples?”

“I don’t fucking know! We just do! Stop talking about my nipples!”

“But they are beings adorable!”

The blond gapes up at him, flushing brightly.

“Hah?!”

“Your nipples are being cute! I am likings them very much! May I be beings given permission to touch them?”

Katsuki sputters loudly, but Kirishima is not taking no for an answer.

With a careful swipe of his hand, he’s hefting Katsuki up, up, up into the air, ignoring his panicked yelps as he’s lifted.

“HEY! PUT ME THE FUCK DOWN!”

“It will only be beings for a moment!” He argues back, using his thumb to press the male back against his palm. He’s very small, and Kirishima feels like he’s holding a children’s doll, only much more fragile, and so very warm.

“You are beings so very small,” he murmurs quietly, careful of speaking too loudly. He may not know much, but he knows that Upper Worlder ears are sensitive to loud sounds.

Katsuki makes some kind of choking noise when he uses the very edge of his claw to drag across his chest, trying to see if he can properly feel his nipple. But claws aren’t much for sensation.

“I am not beings able to feel your nipple, Katsuki,” He murmurs with a deep frown, “but I am noticings that it is very squish squish! Are all Upper Worlder nipples beings like this?”

Katsuki doesn’t answer him, but Kirishima doesn’t really need an answer. He’s more interested in the way his nipple flattens when he presses down, only to spring back up when he lets it go. And despite being so spongy, it seems to be getting harder with every touch! How fascinating!

“Katsuki! Your nipple-!”

His words get caught in his throat when he looks up from Katsuki’s chest to his face. He’s flushed so very red, and Kirishima is now noticing how his chest is heaving with each breath. He’s glaring at him as well, but something about the way his eyes are glittering in the slowly darkening light of the cave combined with his red cheeks makes something curl hot and heavy in Kirishima's belly. He gulps softly, his gills flaring once before settling as he presses down again.

The blond gasps, sharp and breathy as his eyes roll closed, and Kirishima echoes it before exhaling shakily.

“K-Katsuki…?”

“Again,” Katsuki breathes softly, his voice weak and desperate as he arches towards his hand.

Kirishima almost feels light headed when he does it again and the blond makes some sort of whining cry, his head falling back against Kirishima’s palm.

“You are likings this?” he whispers softly, and lets the edge of his claw become the pad of a webbed finger.

Katsuki groans ever louder, his legs spreading, and Kirishima suddenly notices the twitching lump underneath his jeweled fabrics.

“Katsuki, what is beings this?” he asks, flicking the glittering material upwards.

“H-hey-!”

Pretty.

That’s the first word that comes to his mind.

Such a pretty, delicate protrusion coming from in between his legs, flushed a dark pink and leaking something clear from the tip, with two fuzzy orbs hanging underneath it.

From the location of it…

“Katsuki, is this beings one of your claspers?” he asks curiously, dipping down for a closer look.

It looks like one, but there’s only one, and it doesn’t look like there’s another one.

“S'not a clasper, it's a dick , shitty Mer,” Katsuki grumbles softly, his dick twitching as his warm breath washes across his slightly chilled skin.

Dick? He’s never heard of this word.

“What is beings a dick?”

“It’s for s-sex. It goes inside of our females to make babies.”

Oh, well he knows what that is.

Kirishima hums, eyeing the twitching thing as he reaches out to nudge it.

“O-oi! Careful! That's fuckin' sensitive-!” Katsuki snaps, but his words are weak, and he shudders when Kirishima actually makes contact.

He shouldn’t be so surprised when his finger comes away wet, but he is.

It’s just a drop on the very tip of his finger, and for an indescribable reason, he finds himself curious about the taste.

“Katsuki, is this wet from your dick beings eatable?”

“Wh-? I mean, yeah, but-!”

He licks it.

Katsuki squawks, but Kirishima isn’t listening. He’s swirling the taste in his mouth curiously, letting it spread across his tongue. It’s salty, but not in a bad way. Just a subtle thing, underlain with some kind of sweetness that Kirishima will never be able to describe.

He tastes like the sea.

“I am likings it!”

Katsuki just gapes up at him.

“May I be havings more?”

“M-more?!”

“Yes, pleasings!”

Katsuki stares up at him, and then he’s standing, his feet digging into the softness of his palm.

“Alright, but we’re gonna do my way. Now hold me up to your mouth.”

Kirishima does so immediately, causing the blond to stumble as he flies through the air at an almost alarming speed.

“G-good job. Now, stick your tongue out.”

He does so, opening as wide as he can and letting his tongue hang from his mouth.

“Fuck, your teeth are sharp,” Katsuki mumbles, parting his wrap so that his dick is bobbing freely in the air. “Here, closer.”

He leans forward, only flinching slightly when the blond grabs his upper lip with one hand as one of his feet drops down to press against his chest and a hand reaches down to balance on his finger.

“Closer,” he whispers, and Kirishima guides him forward until the entire length of his dick is resting against his large, wet tongue.

“Oh, fuck…”

Kirishima moans softly, his eyes falling half lidded as the taste spreads across his tongue. It’s such a pleasant taste, and he only feels himself get hungrier as Katsuki begins to rock back and forth, dragging himself up and down his tongue.

“Holy fuck, your tongue is so soft,” Katsuki groans, looking down at the image of his dick rubbing across the large, pink organ. “It’s wet too, are Mer tongues all this wet?”

Kirishima mumbles something unintelligible, and Katsuki hisses when his tongue flexes against him, licking across his balls unintentionally in a way that sends shivers down his spine.

“Fuck!”

Katsuki feels hot, stifled in a way that he’s never felt before, and each movement only makes him that much hotter. Everything is so hot. The setting, the company, the company’s hot, flexing tongue that's making heat boil hot and violent in his stomach. He's never done this before either. Has only had the quick pleasures of his hand in the darkest hours of the night. So to be suddenly confronted with this kind of pleasure, curling around his entire being...

He couldn't last if he wanted to.

“I’m cumming!”

Kirishima’s startled by the hot splash of something against his tongue. It tastes so very good, the same salty sweetness that came from Katsuki’s dick before, but this is so much more potent, and thicker, too.

“Fucking shit-!”

the Mer just moans and lets it happen, moaning with each shot of whatever this is that lands on his tongue until it slows to a stop.

Katsuki slumps against his hand, panting heavily, and Kirishima takes his time rolling whatever it is in his mouth happily until it’s all gone.

“I am likings this taste very much,” he chirps.

Katsuki wheezes.

He eases the male onto the stone floor, still licking his lips curiously. He wants more of that taste on his tongue.

“Holy fuck.”

He looks down, but Katsuki is looking down, too, down to where his claspers are peeking out of their slit, leaking their own glowing secretions down the flat front of his tail.

"My claspers," he says proudly, easing them free of their slit until they're bobbing and twitching in the cool air, making Katsuki gulp nervously.

"T-there's two of them."

"Yes. All male Mers are beings having two claspers, but only one is for going on the insides of female. The other is…I'm beings not sure what the other is for. We just have. Still beings pleasurable when touched, but is not servings of real purpose."

“You’re fuckin’ big,” Katsuki whispers.

“Oh! I do not havings to be!”

That makes Katsuki look up.

“You don’t?!”

“No! Here! I am showings you!”

And the Mer begins to glow .

Neon blue lines of color, the same color leaking from the tips of his twin dicks, travel from the very tips of his tail, up his back and down his arms, and then he starts to shrink, until Katsuki is staring up at someone only a foot taller than him rather than a hundred, standing before him with a bright, sharp smile.

“See? Same sizings!”

It’s not quite right. He still has his ears, and he still has his scales, curling around his upper body. He’s standing on two legs, but his feet and hands still have webbing between them, and his claws still top each finger and toe. His dicks are peeking from their slit, and still dripping even though they’re drastically smaller than before, and Katsuki can see the bright pattern of scales surrounding the base.

“I see,” he mumbles stoically, but his arms already reaching up to wrap around the Kirishima's neck, yanking him down into a rough kiss.

It’s clumsy, and Katsuki almost cuts himself on one of his sharp teeth before he manages to slow down, making the kiss a shared pleasure instead of an attack. When they part, both are breathing heavy, and he can see that the Mer’s eyes are blown with lust.

“Oh, wow. What was that?”

“‘S called a kiss. You’ve never kissed before?”

“I’ve never been kissed like that before.”

Katsuki frowns, his head cocking to the side.

“You're talking better than you were a second ago.”

Kirishima nods, his hair swirling over his shoulders. “When Mers connect mucus membranes with other species, we're able to communicate in that language, as well as gain knowledge of the new words we learn. It’s usually only for sea creatures, but apparently it works with Upper Worlders as well. However, the catch is that I am only knowledgeable of the words that you yourself know, and I can't learn more knowledge unless I either learn myself, or kiss someone who has more knowledge than you.”

"So you have to kiss every sea creature or person you come across just to talk to them?"

"Only briefly. Kind of like a quick peck to trade saliva more than an actual kiss."

“How convenient,” he mumbles, reaching up to tuck the hair behind his ear, only for Kirishima to flinch back.

“A-ah. Sorry. My hair isn’t usually like this.”

“Like...hair?”

“What? No! I mean, yes, but like...all limp like this. In the water, it’s all floaty and nice to look at, but out here it’s just flat and boring,” he murmurs, rubbing a shy hand across the back of his neck.

Katsuki frowns, not liking the way that the mershark’s confidence seems to have dimmed, and with a slow drag of his fingers through thick hair as well as a few whispered words of magic, Kirishima’s hair is floating in the air as if he’s underwater.

“There.”

It’s Kirishima’s turn to be surprised, his mouth dropping as his hands fly up to his own head. “You are a magic user?!”

“Of course. My family comes from a long line of mages,” Katsuki scoffs proudly, his nose in the air. “I’ve been practicing since before I could even walk. You didn't see that big ass fireball I came in here with?”

“That's amazing! Thank you, Katsuki!”

He can’t reply because Kirishima is kissing him again, furious and pleased, his tongue sweeping into his open mouth, bringing with it the taste of Katsuki's own semen.

He groans and fists two handfuls of bright red hair, yanking him closer as they both ease to the floor, still warm even though the sun has long since disappeared below the horizon. But they are not completely in the dark.

Kirishima is still glowing softly, the same blue of his transformation to a normal sized being. It’s ridiculously pretty in the darkness, and Katsuki feels a peace than he has not known in many a year.

“Touch me,” he gasps softly, reaching down to grip a clawed hand and raise it to his chest, until the Mer is squeezing his tit firmly.

“Fuck, that’s really soft.” Kirishima whines breathlessly, moving from his lips to his neck as he squeezes the mound of flesh tighter. His claspers are pulsing desperately, having not retreated despite their mundane conversation earlier, and he’s more than eager to put them to use.

“Katsuki,” he moans into their kiss, rutting against his stomach. “Can I, baby?”

The word is foreign on his tongue, weird in this context, but something tells him to say it anyways.

Scorching heat like nothing Katsuki has ever felt rips through him, making him moan loudly until it’s echoing back at them from the walls of the cave.

"Fuck!"

"Oh, you like that," The Mer whispers, licking a slow stripe over his sweaty skin as his other hand reaches down to grip his ass tightly, forcing their hips to grind together in a way that makes sparks briefly light up in his hands.

"Ah!"

"Fuck, you sound pretty," he groans, biting briefly into his shoulder before moving back to his mouth. His hands are moving faster now, parting his smooth thighs and lifting his skirts, because that's what they're called, skirts . "Can't wait to get inside of you."

Katsuki feels like he's on fire, his chest heaving with each gasp for air. Kirishima's dicks (claspers?) are pushing against his own, coating him in thick, sticky precum that's sending tingles shooting down his spine. He feels weak with pleasure, but his legs are still opening, spreading wider to let Kirishima rut harder against him.

"Ah! Ah!"

"That's it, gorgeous," the Mer moans softly, one of his clawed fingers dipping between his cheeks to press against his hole. According to his new knowledge of the human world, his seeding clasper goes here. But it has to be stretched?

"Katsuki, how do I open you for me?" He asks, pressing harder just to feel the blond shudder against him.

He doesn't answer right away, but Kirishima watches curiously as he reaches between his legs with a shaking hand.

"L-like this…"

A few whispered words of magic, and suddenly his entrance is slick and glistening, clenching down on nothing. 

Kirishima's breath catches in his throat.

"Oh…"

"Please," Katsuki whines, reaching up for him desperately. "Please!"

With a bitten off growl of need, he pushes forward, the slick already coating his own cock easing his way forward into the tightest, wettest heat that he's ever had the pleasure of being inside of while his dormant clasper rubs against Katsuki's cock, hot and twitching and flushed just like it was when he used his tongue.

"Fuck-!"

It's almost overwhelming, how such a simple act steals the very breath from Kirishima's lungs, his seeding clasper sliding farther and farther inside until his hips meet the soft globes of Katsuki's ass.

"Ah! Kirish-!"

It isn't soft. It isn't a slow, exploring love making.

This is raw and dirty from the first thrust, hands grabbing hair and claws digging into pale thighs with bruising force, lifting them higher and higher with each rough snap of his hips as Katsuki twists below him, his cries rebounding off the walls of the cave and back to them, creating a carnal medley of noise that neither of them will ever forget.

"Harder," Katsuki gasps, his head cracking against the stone. He knows there's pain, but he can't focus on anything other than his pleasure and the way Kirishima's second dick is rubbing against his own with each thrust, wet with their weird, glowing fluids.

Kirishima is like an animal between his legs, dragging him down every time he thrusts up, scraping his shoulders against the floor of the cave as he chases his orgasm to the very end. He's only had sex with a few of his own kind, and it's never been anything like this. Never something so all consuming, making him gasp desperately for air.

Or maybe that's him actually gasping for air.

He can't breathe.

Has it been so many hours already?

He stops, his body heaving as he stares down as Katsuki with wide, slightly panicked eyes.

"Do you trust me?"

Katsuki is breathing just as hard, his face pinching in confusion.

"What?!"

"Do you trust me, Katsuki?"

And although it hasn't even been a full day since they've met, he nods.

That bright smile is worth it.

It is not worth the way the Mershark wraps his arms around him and rolls , sending them both tumbling off the edge of the stone floor and splashing into the ocean a few feet below them.

Cold.

Shockingly cold.

Not cold enough to freeze, since it's still the middle of the summer months, but definitely cold enough to make him screech angrily right before strong hands drag him fully under the water.

He wants to struggle. Wants to fight. His body isn't made for being underwater, and every instinct of his is telling him to swim for the surface, for safety.

But Kirishima's hands are on Katsuki's cheeks.

His eyes are looking into Katsuki's eyes.

His mouth is forming one word, over and over.

Breathe.

He gasps, and feels water enter his body, before being expelled.

He can breathe.

"H-how…?"

"Another mucus membrane perk," Kirishima says with a grin, pulling him into another kiss.

It's different, being underwater, but they're no less vigorous than before, Kirishima digging the claws of his hands and feet roughly into the stone wall with Katsuki in between so they aren't floating in the middle of the ocean. And since he's used to moving in water, each thrust of his hips is just as hard as he was on land, ripping soft cries from Katsuki's throat as he drags his own blunt nails across the scales of his back.

"A-ah! Ah! Kirishima!"

They're both young and inexperienced. They don't have the stamina to keep going or the skill to hold back.

"I-I'm gonna cum," Kirishima growls, his eyes squeezing shut as he buries his face into the crook of a pale neck.

Katsuki just moans and locks his legs around his waist, pulling him forward each time he pulls back as he drops both hands down to circle his own cock as well as Kirishima's second clasper, squeezing them both tightly between his hands and letting the rough rock of the mershark's hips force them to fuck his fists.

"Inside, do it inside," he gasps, watching with wide eyes as Kirishima's stripes begin to glow brighter and brighter all across his body. "Cum for me, Kirishima."

With a snarl and a bite into Katsuki's bared shoulder, he does just that, his orgasm ripping through him with the force of a typhoon, hips shaking as he shoves himself as deep as he can.

The sudden swell at the base of his cock is a surprise to both of them.

"Whoa!"

"Ah!"

And suddenly it's so much better . Katsuki is squeezing down onto the knot that had just swelled at the base of his seeding clasper, clenching so tightly that he feels light headed with it, his hips jumping erratically.

Katsuki follows him over the edge with a choked cry, his eyes rolling into the back of his head as he arches off the wall and his seed spurts into the open sea to be washed away by the tide. He can't stop it, his body jolting each time the hard base of Kirishima's cock grinds against his prostate, forcing him into an elongated orgasm.

The intensity leave them both shaking in each other's arms, staring wide eyes but unseeing at whatever's in front of them.

"Wow," Katsuki finally whispers.

Kirishima grunts, gently dislodging his teeth from the blond's shoulder.

"Y-yeah…"

"I mean wow."

They just breathe together for a while, foreheads pressed against each other before Kirishima finally pushes away from the wall, back up towards the surface.

They don't emerge right away, for obvious reasons, but instead just float quietly, staring at the stars twinkling in the sky until Kirishima's knot softens enough for him to pull out, sending thick lines of blue glowing liquid down his still shaking thighs into the water.

"That's so weird."

"I am sorry about my knot," Kirishima whispers, his ears low against his head and hair no longer held by the magic that Katsuki had cast on in. "I didn't know that I would...do that."

"Don't worry about it," Katsuki sighs contentedly, smiling for the first time since they had met.

He's ridiculously beautiful.

"D-do you think that we could meet again?" He whispers, looking at him with pleading red eyes.

Katsuki hums softly, washing their spends away in the water until his skirt is clinging to his legs like a second skin.

"That depends," he murmurs, meeting the Mershark's eyes with a small smile. "You gonna be here tomorrow?"

Kirishima's smile is blinding.