Sirius Black
Sirius Black was raised with these facts drilled into him.
One: He was Sirius Black of House Black, one of the most ancient siren families in the great seas.
Two: Soulmates existed, and the bond between a siren and their soulmate is the strongest and most important things in the world
Three: His parents would never allow him the freedom to find his soulmate.
They were purists of the worst kind. They were the sort that believed a soulmate isn't good enough if not from a respectable enough family. They'd checked, and he wasn't mated with anyone from the families they liked, so they decided he'd marry some siren woman they chose.
Sirius’s brother, Regulus, was lucky enough to find his soulmate. The ever important Batty Crouch. *Cough* Barty *Cough*. Okay, so Sirius didn't like Barty all that much. He thought the guy was crazy. But in the end, he treated Regulus like royalty didn't he? Sirius was jealous at times, I mean, Reggie was only 9, how come he already knows his soulmate? Sirius would wonder sometimes.
So Sirius would disappear from their coven's caves, Narcissa and Regulus would cover for him when he adventured. At some point, he found a beautiful alcove hidden between the ocean and the rocky beaches at the shore of a boarding school. He'd never seen any of the kids wander out that far, but they were still near enough he could discreetly watch and listen when they chatted.
When Sirius was eleven, he'd wandered up to the alcove a little after the school year started, and he saw a human boy. One with messy black curls for hair, tanned skin and brown eyes. He looked like he was about the same age as Sirius, and he was drawn. He would watch as the loud boisterous boy wandered near his alcove, usually chatting loudly with some other students.
During a spring break at the school, James (Sirius had learned his name after watching him for so long) had decided to stay at Hogwarts. There weren't as many students around, which led to James exploring a little more, and discovering Sirius's alcove.
When James first found it, Sirius wasn't there, but it wasn't even half an hour later before Sirius swam in quietly, startling the other boy. Sirius startled too, staring wide eyed at the boy in silence, until James bravely broke it.
“H-Hi?” He stuttered out. Sirius started to move backwards, an attempt to swim away he thinks, when James gasped out, “Wait! Don't leave!”
Oddly enough, Sirius listens, and actually swims closer to the human. “W-What are you? A mermaid?” James asked quietly, his eyes taking in the appearance of Sirius. Sirius had long black hair, just below his shoulders, and pale skin. His skin was almost blue if you looked too hard. He had stormy grey eyes, and his tail was.. a beautiful deep black color.
“I prefer the term Siren actually. But my parents say we're a nameless species, too old for anyone to put a title on.” Sirius informed after a moment.
James seemed to accept that information, before asking, “Do you have a name then?” His voice was unsure of the question, for only a moment.
“Sirius. Sirius Black.” He said, “What's yours?”
“James Potter.”
After that introduction, a bond had been formed between the two boys. Over the next five years, Sirius was spending as much time up at the boarding school as he could. During the winter breaks and summers, he would be home more since James always spent that time with his parents, but any other time… well the two boys knew everything about each other.
Sirius knew about James’s love for Lily Evans, a beautiful redheaded girl he met at school, and James knew about all of Sirius's home issues. After a while, Sirius had theories that James was his soulmate. He felt drawn to the boy in a way he'd never felt before, but he was thrown off, because he never felt romantic attraction towards him. It didn't matter how old they got, or how attractive James got… he never felt it.
When he told Regulus of his thoughts, and his new friend, Regulus did a bunch of research. He looked at the records kept by the generations and generations of sirens. Somehow, they had a sort of magic that gave them waterproof pen and paper, to be able to write and track things as well as any human.
From said research, Regulus discovered it's possible to have a platonic soulmate. It doesn't happen often, but it's very possible for a siren to meet someone that they click with on a level lower than romance but higher than friendship. Sirius accepted that as it was, and continued his routine of meeting with James.
During James’s sixth year, he brought friends. Two to be exact. Peter Pettigrew and.. Remus Lupin. As soon as Sirius and Remus met eyes, he knew. He knew what Remus meant to him. This was his soulmate. His bonded. It seemed like Remus felt it too, the pull, but they both resisted.
When Sirius went to Regulus again, distraught and feeling heavy without his soulmate, Regulus cried. He cried because he knew he'd be losing his only brother.
“Go be with him, Sirius. Leave this place, where you're so miserable, and be with your love..” Regulus hugged his brother desperately, Narcissa jumping into the hug as she knew she'd miss her favorite cousin.
From that moment, Sirius knew what he had to do. When a siren had a soulmate that was human, either the human had to give up their humanity to become a siren, or a siren had to give up their nature to become human. A sacrifice in the honor of the bond. Neither could take it by force, it had to be willingly given.
By this point, Sirius had known Remus for a month. That was all he needed. He met his friends at the alcove, looking frantic. He explained everything, his soulmate bonds (yes, James was still his soulmate platonically, while Remus was just his).
James and Peter left, and Sirius told Remus what he had to ask, “Are you willing to give up your nature for me, Sirius?” He asked gently. His voice was always gentle and addicting. He found never ending comfort in it, every time he heard it.
“Yes. I am. I'd give anything for you Moony..” He didn't hesitate as he said it, all the fear and anxiety he held about leaving behind his family was gone, replaced instantly with the need to be with his soulmate.
Like a tug at their souls, the two boys felt themselves being tugged to each other, until their lips met in an awkward kiss. At least, it started awkward, but soon enough it became passionate and loving. Before Sirius could process what was happening, Remus had pulled him from the water, onto the land, and held him as they kissed, Remus burying his hand in Sirius’s hair.
When the two boys pulled apart breathlessly, Sirius was turned human. He had human legs.. and was very apparently naked. Which James had thought of, cause not even a minute later, James and Peter jogged back in with someone James underwear and old gym shorts.
It took some time for Sirius to adjust, but the Potters took him in, and he experienced a loving home for the first time. Euphemia, or Effie as she liked to be called, was a very gentle mother. She'd given birth to James at forty, so it was a miracle they had kids at all, and they treated Sirius like one of their own immediately. Fleamont, or Monty, was just as loving, in a fatherly way.
Sirius melded into James’s friend groups easily, finally meeting the famous Lily, who by this point got over her childhood hatred for James and started being friendly with him.
A few years later, Sirius was nineteen, and was watching as his best friend got married to the love of his life, with his own love beside him. Remus would lean down, whispering promises to marry him someday. “That'll be us before you know it Siri, just wait..” his honeyed promises sat well in Sirius's ears.
Then, he was introduced to Harry James Potter, his godson. And he sobbed like a kid when he held the boy the first time. He just knew Harry would be just like him someday.
Sirius married Remus before long, and it was about time wasn't it? It was a small wedding, only Pete, Effie and Monty, and Lily and James were there to see it. But it was exactly what they wanted.
Sirius felt a part of him break when James died. It happened because of a psychopath who found his way to their small town. He broke into the Potter's house, killing James and Lily, and was forced to flee the scene before he could get to Harry. Sirius tried to get custody of his Godson, but the officials in charge of the case were bigots, and didn't much appreciate giving Harry to a gay couple.
When Harry was placed with the Weasley's, a well known family in the area, he was relieved. They were friends, family really. He and Remus were given every opportunity to see Harry, even moving closer to them.
When Harry was ten, and unexplainably survived the attack on his life… Sirius knew that they were being watched, and protected. And when he heard his godson's description of the woman… he knew almost immediately who it was.
Sometimes, he would sit on a pier with his feet in the water. When he did, he whispered gentle thanks to his beloved cousin, not knowing if she could hear him at all.
When Harry told everyone that he had to go sail the seas, Sirius knew. Harry told everyone that he had to find himself, and Sirius knew exactly what he meant. He knew, because Sirius was the same way. That night, Sirius sat on the same pier, and prayed that his brother, his cousin, his friends, would take care of Harry for him.