Growing up… Katsuki always believe that humans were scum and needed to die, his mother Mitsuki who was the queen of there kingdom told him that while most humans were scum… some were actually really nice but were mistreated for no good reason… she then told him that one day he would meet a human like that, and of course Katsuki didn't believe her and swam away… while they were merfolk and were stronger then the humans… sometimes there people died because the humans surprised them with something they have which causes there people problems… and he hated them even more then before.
His father Masaru, tried his best to help Katsuki understand but nothing worked… it was clear to them that in order for Katsuki to believe them… he would need to meet a human himself, so one night they swam to the surface and shifted into human forms… well as human as they can be with there webbed hands and scales by there necks and faces, they got dressed in the clothes some of there people who live on the surface hid for them and began to wonder around, all Katsuki saw was abuse, theft and pain… nothing else that was good until he saw her.
It was a young girl rushing across the street to help a poor man by giving him food and some money, she looked thin and dirty yet she helped the man instead of herself which shocked Katsuki greatly as she then left the man… and Katsuki chased after her, he wanted to know more about her and where she lived, his parents were trying to catch up to him but he was too fast for them right now, after a while though… he lost her, he didn't know where the girl went and began to walk around in the hopes of finding her… but no luck, soon his parents caught up to him just as he heard a scream and he ran to locate the sound.
There he found the girl getting beaten up by a man who looked well dressed… with a woman and a little girl who looked like the girl he was beating up smiling towards her pain and suffering…
"You damn bitch! Stealing from us and stealing Ochako's favorite necklace! Your going to regret it by dying out here while we forget you ever lived!" he shouted as he kicked her a few times before slamming her in a brick wall and then leaving with the other two, Katsuki was horrified by the treatment the girl was getting when she clearly did nothing wrong… his parents arrived as they found him kneeling before the girl who was doing her best to stay awake right now… it was clear that if she didn't get help right now she would die "What's your name?" he asked making the girl blink a few times "Jasmine… Uraraka" she whimpered out.
Katsuki nodded as he pulled off a necklace with gorgeous black and orange pearls with a black and orange clam in the middle, his parents watched in shock what he was doing but neither tried to stop him or spoke up as he placed the necklace around her neck "My name is Katsuki Bakugo… I am the future king of the merfolk and one day I will return to make you my wife and queen" he explained as she watched her in a slight daze "Until then you will heal, be safe and well cared for by many and you will never feel pain again" he added before he watched her black out, Masaru took him away and were back in the water with Mitsuki catching up to them a moment later… he was told that what he did was a promise to Jasmine and he can't break it "If you were to break it one day… you will turn into bubbles and never be able to be reborn" his mother explained making him nod as they returned home.
And Katsuki began to wait for the next eight years where he would turn eighteen and be able to claim Jasmine as his bride…
