TLDR: Think of mutants from X-Men and Sirens from Borderlands in our world with sex.
(I know it's not that good, so any advice or constructive criticism would be nice.)
No one knows where exactly they came from. The very first officially recorded Siren appeared in 1968 as she saved a commercial flight from crashing in Oregon. The young woman was from the United States, she had no answers to her kind's origin. Two years later more Sirens started to appear and they were from the United States as well. It wasn't until another year or two that Sirens from other countries appeared.
How to determine if one is a Siren. They're always women, incredibly beautiful, have powers, and high libido. Sirens don't show their abilities and high libido until after puberty, 18. The power and libido don't always appear right on their 18th birthday, as it might take a few days to a week.
A Siren's power isn't specifically unique to them. Common powers are telekinesis, super strength and durability, telepath, and illusions. Some Sirens can have stronger versions of these powers. For example, most telekinetics can only move small objects, but very few can move large objects. Also, telepaths don't have the same powers as Emma Frost. The most they can do is just read another person's mind and communicate through thoughts.
Other abilities that all Sirens have are immense stamina (during sexual activities), immunity to all sickness and diseases, and possibly a longer lifespan.
Sirens can keep control of their libido, but it is unhealthy for them. They can become weaker and die if they don't don't have an orgasm. Luckily they can easily cum through many possibilities. Drugs and alcohol do affect Sirens the same as any other human beings. Orgasms also purify a Siren's body from any sort of effect from drugs and alcohol. This was found out from the first Siren, as she had to go through a lot during her first two years alone.
Sexual activities with other people give Sirens more powerful orgasms than with themselves. Sirens are willing to have sex with anyone or anything and do not find people who haven't finished going through puberty to be sexually attractive.
The world sees Sirens as angels and demons. Most people appreciate their contribution to the world, especially the sex. Some others find them to be dangerous and their sexual behavior to be repulsive. Sex isn't viewed too poorly (better than our world, but still not positive) by most of the world, thanks to Sirens. Although rape seems to have spiked a little more compared to our world. Since all Sirens are women and love sex, people would rape more women and calling them Sirens to justify that it was just rape-play. It also doesn't help that some Sirens, who don't want to admit that they're Sirens, end up raping people due to denying their libido. Raping an actual Siren is still illegal despite their willingness eventually coming out during the rape. Telepathic Sirens have to come in for any rape case involving Sirens and victims claimed to be Sirens.
Most Sirens live on a large island in the Pacific Ocean, called Pothe. The island is considered as its own country. It has its own high school, for seniors only, and University. The island is also a tourist spot, for obvious reasons. When a girl reaches 18 and becomes a Siren, a letter is sent to them detailing how they can come to Pothe. New Sirens on Pothe learn how to stop their abilities from going wild, control their libido, and continue their education. The Sirens that are citizens of Pothe are usually far more capable in combat and with their abilities than other Sirens, in case anyone was stupid enough to invade.
Sirens are not allowed to join any country's military, though this never stopped countries from trying to secretly recruit Sirens. There are also rogue Sirens around the world that are vigilantes or criminals. Police forces around the world are equipped to deal with most Sirens, but there are some cities with a squad of Sirens. These Sirens, however, don't have the training as citizens of Pothe.
Asani Lynn is the main character but will have more characters to play with, some will just be regular people.