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Izuku is one of the few mermen of his clan to explore the surface of the ocean. His mother is usually worried about him getting in trouble, but so far, he has learned a couple of things about the human world that fascinate him.
Katsuki, his best friend, belongs to one of the strongest clans; he's very proud of it and always assures Izuku he's the only one who can protect him.
When they were kids, Izuku assured him he could protect himself and got into an argument with the blond that made them stay away from each other for almost a month.
But then, little mer Katsuki paid him a visit, apologized for his behavior, and told Izuku that even if he knew he could protect himself, Katsuki still wanted to be there for him.
After that day, they became inseparable.
It's been a while since that, and now they have reached the proper age to be considered adults.
Katsuki, just like Izuku's mother, doesn't like him going to the surface that often; he doesn't trust humans and believes Izuku should be more careful.
Despite that, the blond merman has followed Izuku a couple of times, mostly to keep an eye on him, and make sure they don't get spotted by humans.
"I think they're friendly, Kacchan," Izuku mumbles, feeling his friend's grip on his arm to prevent him from ascending even more.
"They'll catch you and hurt you, Izuku," Katsuki growls in response, pulling the green haired merman into his arms. "I don't want anything to happen to you."
Izuku can't help but melt whenever Katsuki shows his vulnerable side to him, so he often gives in.
"Alright, Kacchan. Let's go back."
With a firm hand grabbing him by the waist, Katsuki happily leads him deep into the ocean and back home.
They catch a couple of small fish to feed their families, and Katsuki makes it into a competition that he, of course, ends up winning.
However, he gives a bunch of his own fish to Izuku.
"Kacchan, there's no need-"
"Please take them. I want to show you that I can provide for you, nerd," Katsuki looks uncharacteristically flustered then, rubbing the back of his head repeatedly. "I know you can do it, but I still want to give you those."
Izuku knows he's being sincere, so he gladly accepts the gift, although he feels confused after that.
Then Katsuki starts acting out of the ordinary and giving him all kinds of beautiful shells; he even gave Izuku a pearl once.
But that's not exactly what puzzles him the most, it's the fact that Katsuki swims a couple of circles around him whenever Izuku is near another mermen or merwomen, especially if they seem eager to talk to him.
"We'll never make more friends like that, Kacchan," he mumbles once, watching as another merman recoils as soon as he sees the blond swimming in circles around Izuku.
Sometimes, Katsuki even hisses at them.
Izuku tries not to roll his eyes.
"We already have friends!" Katsuki retorts, embracing Izuku.
"That doesn't mean we can't make more," Izuku insists stubbornly.
"We don't need them. Besides, they didn't want to be your friends," Katsuki assures him, narrowing his eyes at the thought.
"Alright," Izuku sighs. "Let's go all the way back; Mom must be worried about me."
Katsuki smirks, very pleased with himself, before nuzzling against Izuku's cheek until he makes him chuckle.
Even though he enjoys physical affection, he can't help but wonder if other mer folk don't get a bit confused by his attitude.
Someone could misunderstand the situation and think Katsuki wants Izuku as his mate, which is definitely not the case.
It's true they're friends, but Izuku knows Katsuki can find someone better in his own clan.
Sometimes, he gets a bit sad thinking about that; they're just not meant to be.
However, Izuku is glad to know that Katsuki cares deeply about him, even though they don't agree on what "danger" truly is most of the time. That's why the green haired merman starts swimming towards the surface whenever Katsuki has to do an activity with the members of his clan.
After a few weeks, Izuku meets a very kind and nice human woman named Uraraka Ochako; he saves her from drowning and she's so grateful that even though it takes her a while to accept that apparently mer-folk are real, she eventually gets over the shock and agrees to be his friend.
They meet every few days or so, near the spot where they met; she always comes in her tiny boat, alone, but happy to see him nonetheless.
Izuku answers all the questions she has about the ocean, and she teaches him new things about human inventions and the way they communicate with each other.
He's fascinated by the things she calls cellphones and how they work. He looks in awe at all the pictures as she has taken; Izuku can't believe it at first; it's like she has the ability to see her reflexion frozen in different places and different times even though she's not longer there anymore.
He manages to keep his friend a secret for a while, but Katsuki figures out that there's something weird about Izuku.
His friend with explosive attitude narrows his eyes, crosses his arms over his broad chest and asks him what's going on. Izuku tells him that everything's fine, that nothing has changed, but he has the feeling that the answer doesn't convince Katsuki at all.
Surprisingly though, Katsuki doesn't insist.
Instead, he follows Izuku one day.
"Where the hell do you think you're going?" Katsuki growls, reaching him in the blink of an eye, grabbing his arm before Izuku can move away. "The surface is dangerous!"
"Not this part," Izuku assures him. "It's always empty. I've been there a lot of times."
Katsuki sighs and pinches the bridge of his nose, it's obvious that he's irritated or at least he tries to be because he narrows his eyes at Izuku before calming down.
"Why didn't you tell me?"
"Because you wouldn't have let me see my friend!"
"Friend?" Katsuki's left eye twitches for a moment, before frowning again. "What's his name?"
"Her name is Uraraka," Izuku beams. "And she's a human."
Katsuki doesn't react that well to the news, but he does reluctantly follow Izuku, not because he wants to meet that human, but because he doesn't trust her and he wants to be there to save his reckless friend.
"Uraraka!" Izuku grins, face popping out of the water. "This is Kacchan!"
She smiles despite finding herself glared at by another merman, only that he's bigger, stronger and definitely not as kind as her friend Izuku. This one seems dangerous, but she's not afraid of him.
"Nice to meet you, Kacchan!"
"Don't call me that!" the blond merman growls. "Only Izuku can."
"What's your name then?"
"Bakugo Katsuki," he mumbles reluctantly, after the green haired merman elbows him on the ribs.
After a while of talking, Izuku can tell that Katsuki has realized that Uraraka is definitely not their enemy and clearly hasn't told another human about Izuku.
However, he still doesn't like her, especially when she leans over her boat to touch Izuku.
"Kacchan, stop hissing at her! I apologize on his behalf, Uraraka, he's not always like this..."
"It's okay!" She chuckles. "I get it, he's jealous and possessive."
That confuses Izuku a lot, especially when Katsuki doesn't deny it.
"Don't worry! Midoriya and I are just friends. He's all yours."
Katsuki relaxes a lot after that, nodding at her.
"What do you mean, Uraraka?"
"She knows you're my mate," Katsuki says, clearly proud, prompting Izuku to blush to the tips of his ears.
"What? Your mate?"
"Well, not yet, but soon." The blond merman clarifies, still smirking, however, that smug expression on his face vanishes in a second, when he realizes Izuku is clearly confused. "I've been courting you for a few months now, Izuku, come on!"
"I-I didn't know, Kacchan! But I'm really glad your are, so please don't stop. I'd like to be your mate!" Izuku tears up a bit, clearly happy.
Katsuki melts almost immediately and pulls him into his arms, nuzzling agaisnt his cheek.
"You two are so cute!" Uraraka coos, happy for them.
"This is a private moment, Round face!" Katsuki huffs, making Izuku chuckle.
"You are the ones with their head out of the water!"
"Goodbye, Uraraka!" Izuku wakes, still giggling as Katsuki pulls him under and deep into the ocean.
"See you in a week!"
After they're both completely inside, Katsuki presses their foreheads together and tells him how much he loves him.
Izuku is sure they're going to be really happy together.