Chapter Text
Kirishima was sad to lose his friend. Katsuki was his oldest friend in the world. He had been the person to help him through his mother’s disappearance and he was the man he had taken a vow to protect.
He had taken over nightwatch the next night and this would be the last time he ever saw his friend.
He left the captain’s quarters first to make sure the deck was empty. It was the middle of the night and the crew had gone under several hours before to sleep.
“Are you sure, Katsuki?” Midoriya asked, caressing his cheek.
“I’m sure.” he gave his own soft smile. Midoriya nodded and pulled himself over the rim of the tank, careful not to make any other thumps with his heavy tail. Katsuki walked ahead of him, opening the door and with a wave of Kirishima’s hand they stepped out onto the deck. The last time Midoriya was here he had been attacked and tranquilized, the memory made his heart pound.
“Izuku is going to make a loud splash when he goes over so you go first.” he gripped Katsuki’s shoulder. The prince looked at his friend and gave him a genuine smile. He pulled him into a tight hug and Kirishima returned it shakey, trying to keep the tears at bay.
“I’m gonna miss you, man. But I'm happy if you’re happy.” He pulled back and squeezed his shoulders. “Take care of yourself.” he gave a watery grin.
“You too. I'll miss you most of all.” he gave him a small rare smile.
Katsuki opened the swinging rail to the ladder built into the side of the ship. He slowly started making his way down and stopped right before the edge of the water. The waves were not too bad, gently lapping at the hull and wetting his bare feet. He clung to the side and looked up at the edge.
“Take care of him, Izuku.” he squeezed the merman’s arm and Midoriya nodded with a small smile.
“Thank you.” and he fell overboard. Kirihsima closed the small gate and relatched it, looking over the side to give his friend one last wave before the blonde let go and disappeared into the sea.
*****
The water was cold, freezing cold and he felt his body tense all at once. His breathing picked up and he felt Midoriya’s warm hands come up to help him out of the only warmth he had.
“It’s okay, Katsuki. You’ll get used to it.” he smiled. His skin looked brighter and his hair was shining, his eyes glowing under the moonlight. With some difficulty they managed to get him out of all his clothes. He shivered from the cold and Midoriya cupped his cheek.
“You ready?” he asked and Katsuki nodded. “Take a deep breath.” he sucked in a gulp of air and held it before Midoriya dragged him under. He pulled him some ways down until his ears started to hurt and then the merman stopped. He came up to hold him and cupped his cheeks.
“ I love you, Katsuki. ” He could hear him clearly and he mouthed the words back to him.
“ Stay with me. ” Always.
Midoriya kissed him, opening his mouth and breathing new life into his lungs. Katsuki could feel the cold fading until he was comfortable and warm. He opened his eyes as he drew away and everything looked brighter. They weren’t surrounded by darkness anymore and Midoriya turned gently tugging on his hand. Katsuki began to swim after him. Kicking his legs and pushing the water with his arms until he felt it getting easier. His legs naturally stopped kicking apart and he felt the water rushing passed him. He could feel a strange sensation in his ears and nails, his teeth ached but he paid it no mind. Content to dance in the water with his merman with the ease of his swimming now. He found it easy to change directions and no longer needed to use his arms. They twirled together and he basked in the happiness they were both glowing with. Finally the loud rushing in his ears faded until all he could hear was the song Midoriya sang. A song of joy and love instead of the grief he heard before.
Katsuki opened his mouth and the words flowed out. A language he didn't recognize but the words felt familiar. Midoriya raised his hands before throwing them down and spinning upwards, Katsuki following him closely.
As he sang he began to understand the lyrics. Each new word becoming clear and he felt a piece of himself he didn't know was missing join him.
Finally their voices faded until Katsuki rested in front of him. Izuku’s eyes were shiny as he looked him over.
“ Congratulations, Katsuki. ” he looked down and gasped. His legs were gone, replaced by a much longer tail. His scales were perfect and shiny, a deep red like rubies with a wide fin at the end, black as the deep sea and joined by smaller fins along his sides. His chest didn’t rise and fall like it normally would, instead he saw three long slits on either side of his ribcage, gently parting and closing with his normal pace. He flicked his tongue over his teeth and found them razor sharp, the appendage felt a bit stiffer, more sturdy to not be cut.
Midoriya swam down a little and traced his dark scales.
“ Beautiful.” he rose back up and kissed him again. Letting their lips mold together and enjoying the feeling of Katsuki’s fingers in his hair.
“ My beautiful merman.” Katsuki whispered against his lips and paused realizing his tongue was different.
“ When you change, you're blessed with the original tongue and lasting life. ”
“ Lasting life?”
“We are tied. For as long as I live, so shall you.” he smiled again. “ Come, there is so much to teach you.” he grinned and they dived deeper.
“ You are now part of the ocean, feel the eb and flow. Reach out to find the creatures around you .” he instructed, watching as Katsuki paused to close his eyes. He wasn't sure how to do this but he just focused and the skill came naturally.
“ You’ll learn how to live like me. How to fight like me. And..how to mate like me.” he blushed and Katsuki felt his blood stir at the implications.
As they journeyed, Katsuki learned. Each place they went he was taught the creatures that lived there, how to hunt, how to truly swim and where to find shelter. While they were strong they still needed rest. In a pod, there would be shifts to keep watch but with only the two of them it was more difficult to rest. They needed a secure place that they could defend.
“ Can you feel the pull?” he asked. “ It's unsettling, isnt it? It's where the mating grounds are, our bodies naturally draw us there every year. The trip is usually a month there and a month back. With our pace and from where we left the ship, we should get there in time.” he smiled.
“ Will you take another?” Katsuki asked, gripping his hand even as they swam.
“ Never. Merfolk mate for life.” he grinned and Katsuki returned his smile.
*****
It was another week before they reached the mating grounds and Katsuki gaped in amazement.
On the ocean floor in the darkest depths it was warm with fluorescent plant life clinging to almost the entire bottom of the reef. A variety of fish swam around and while the scenery was gorgeous what was truly breathtaking was the sheer amount of merfolk gathered. There were thousands. All different shapes, sizes and colors. There were darker shades of greens, blues purples while others were near reflective in brightness. Sparkly gems decorated a few groups while others were bare and some wore accessories made of shell or coral. Some wore their hair long and some wore shaved heads and everything in between.
Midoriya pulled on his hand to catch his attention.
“I’m sure it must be a sight but I want to find my pod, come on!” he grinned and they went off searching for his family. They swam for 30 minutes looking around before a cry broke through the chatter. Others seemed unfazed but Midoriya perked up immediately, head swiveling to find the source.
A little mergirl broke through the crowd and threw herself at his mate.
“Deku!” she cried, sobbing into his shoulder.
“Eri!” he giggled happily and cuddled her close. Katsuki watched in contentment. It was like a breath of fresh air to see him so happy. Once she calmed enough he caught her attention.
“Eri, I want you to meet Katsuki” he swam up to introduce them, “he’s going to be my mate.” he smiled and Eri looked at him in wonder.
“Wow..you’re pretty.” she fawned and he felt his cheeks heat a bit.
“Thank you. It’s nice to meet you Eri.” he ruffled her flowy hair a bit and she giggled.
“Where is the pod, can you bring me to them?” Midoriya asked and she sparkled in happiness, tugging on his hand.
“Everyone was so worried! We thought you died!” she shouted while the two mermen swam after her.
“I found Deku!” she shouted as soon as they broke through a group of shallow water merfolk.
A large group of heads swiveled towards them before shouts and cries were heard. Everyone surrounded him and they all cried together, joyous to be reunited.
“Izuku.” one merman in particular swam forwards to greet him and the rest gave space. Katsuki felt his skin itch in anxiety. Something was wrong. He pushed through the small wall between himself and his mate and came to rest at his side sliding a hand to hold around his hips.
The merman was similar in size to Katsuki with two toned hair and a flat expression. His voice was woefully monotone but he looked strong, he looked like a warrior. Like Midoriya.
“I’m so sorry I couldn't save you.” his brows furrowed and he looked shameful for a moment before it passed. “But I believe the gods brought you back for me. I wish to be your mate. I had planned to ask once we arrived before the incident in the west sea. Please accept my courtship.” he held out a beautifully crafted necklace of shells and pearls. Obviously time was put into this gift and Katsuki unintentionally let out a low growl. Midoriya put his hand over the one resting on his hip to calm him.
“I’m honored, Todoroki but I'm afraid I have already found a mate.” he looked genuinely sorry for the refusal and Katsuki relaxed.
“How?” he looked utterly baffled by the thought that he could have found another. Katsuki didn’t know how he hadn’t seen him yet, but decided to make it obvious. He floated behind his mate and wrapped his arms around his waist, careful not to put pressure on his gills, and rested his chin on his shoulder.
“This is Katsuki and he is to be my mate.” he raised a hand to caress his cheek for a moment.
“Nice to meet you.” he offered and Todoroki’s eyes darkened.
“I’ve never seen you before.” he said slowly.
“I’m new.” he shrugged and Midoriya let out a sigh.
“Alright. Katsuki was on the ship that took me.” he grumbled and shouts erupted at the scandal.
“You bonded to him?” Todoroki looked betrayed and Midoriya frowned at him.
“Yes, I love him, and he loves me. That’s how the spell works, you know that. Please don't try to fight it, this is what I want.”
Todoroki looked conflicted but after having a brief stare off he relaxed and sighed.
“It’s a shame but I accept your rejection.” he swam away and the celebration continued. The first day was spent just meeting everyone in his new family and telling the story of how they fell in love. By the end of the day, not that they could really tell this far down, they went to find their own cave. The large mating grounds was at the bottom of a natural valley, surrounded on all sides by deep caves, thousands of them. More than enough for all the merfolk to have their own. Midoriya guided Katsuki to a cave a bit farther than needed and they swam inside. It was a good size with the walls covered in soft seagrass.
“Katsuki.” Midoriya called and laid on his back. “Come here please.” Katsuki swam up to rest above him, their bodies touching from stomach to tail tips. Katsuki pulled him into a long slow kiss. Soft lips pressed to each other, massaging slowly in a patient rhythm.
“Izuku, I love you. Be my mate?” he whispered against his neck. After hearing that Midoriya was to be mated. His mother pulled him aside and gave her blessing but she also told him what to do and what to say. How to complete the courtship.
“Yes.” he moaned softly feeling those same lips press and graze against his neck before kissing his pulse point. He gripped the greenette’s hips tightly before opening his mouth and letting his teeth sink into the juncture of his neck and shoulder. Katsuki felt the bond pulse between them and it sent rushes of electricity up and down his spine like a heartbeat.
“Katsuki.” he gasped and Katsuki released his neck to lick the wound closed.
“My love.” he hummed into his cheek and Midoriya whined, pulling him down so he could lay down his own mark. The same rush came over him but this time packed full of arousal that slammed into his gut and urged him to dominate and breed.
“Izuku.” he growled into his skin. Midoriya’s breath hitched for a second.
“ Take me.”