Hey, I don't own any parts of Harry Potter just the character that I create. :P
Mermaids and Sirens were not too far alike than some think. In fact, they were basically the same species. Only, Sirens were pure, with non spoiled blood. Mermaids, were the offspring of Sirens mating with witches, wizards or muggles. Disgusting things Mermaids were. Mostly, they were born with no powers, only the power to grow a tail in water and breathe on both land and water. Maybe, the rare Mermaid was born and had they had powers, but the warrior Sirens would know of this and exterminate them. As you can see, Sirens detested Mermaids, especially the Sirens that had soiled their precious blood. Sirens, were strong powerful beings. They had the same basic powers of Mermaids, plus more. They had the ability to lure anyone they wanted, to them, simply by singing. Plus, their magic was slightly more advanced than witches and wizards, if the Siren exercised their magic. If not, then it was of basically no use. If practiced, then the Siren would simply have to think of what they wanted to happen, and then wave a hand or nod their head.
Sirens and Mermaids were taught from when they were young, that they were enemies of the other. They could never trust one another, rightfully so. But, it would be hard to distinguish a Mermaid from a Siren. Then again, if you truly studied them, then you would see the differences. Mermaids were beautiful, very much so, but not as much as a Siren. Sirens, were strikingly beautiful, very rarely was there a flaw on their true form or their land form.
So, as you can tell, the sides of the war they were on. Mermaids, were of course on the light side, as that was only by whom they were accepted. Sirens, were obviously, on the dark side, as they were strong and powerful, something craved by the Dark Lord. So, while Mermaids trained to be strong physically. Sirens trained to be strong physically as well as mentally.
Most people would think that the parent of the offspring, would stay and take care of it. But this is not so for Mermaids and Sirens. When the child was born, they were taken to the Black Lake, by Hogwarts, it was the only moderately safe place for their kind. But, even so young, they knew not to associate with the other. When they reached a certain age, their respective side of war they were on, would come and collect them. As they grew before this time, they would practice their strengths. Readying themselves for the day their master would collect them and give them a true role in life. So, it was safe to say, the Black Lake was a very dangerous place for the students of Hogwarts to be in.
This tale, is of one Siren. If you think this is a story, where she befriends a Mermaid, falsifying the legends of her people, then think again. If you're hoping, that she miraculously goes to the light side of the war, and helps them win, then find a different tale. This tale, is of how things should have happened.