"You should talk! Keeping all the babes to yourself, totally ice-blocking my game!" ~Ice Queen from Adventure Time
Elsa is a creation/servant of Khione but she lies to everybody and says that she's been created by Boreas since she hates the guts outta her creator. Elsa physically looks like a 16 year old and is now back in camp. She isn't happy about that but those were Boreas' orders.
Elsa is a mysterious type of girl. Everything about her is a secret even her history and who her creator is. But she covers that up by being a b*tch and shutting her self from the world and by lying.
Real: My name is Elsa and I was created by Khione to do most of her dirty work and be her slave. A couple of others were created too. As an ice nymph I have ice powers but I am very different from most ice nymphs/spirits. My powers, for some reason, are controlled by emotion and they are very hard to control. But I had to keep that hidden or else who knew what Khione would have done in the start. So as I kept that fact hidden, I noticed that when I was at a neutral state, my powers didn't go crazy and I stayed that way for a very long time. I was known as Elsa the emotionless ice nymph since then. I served Khione (more like her slave than servant) for many years and believe me, she has issues that are unsolved. Khione treated me and her other... servants very badly and often punished us if we didn't get things done on time or if we did a mistake (no matter how big or small). Everybody either got angry, or cried. I just remained emotionless because if I set off my powers, I probably would hurt somebody. Then one day, Khione was yelling at me and that's when I couldn't take it any more. I got very very angry and my powers set off. I just waved my hand and ice started forming everywhere. A blizzard came up and most of the things froze because of it. The nymphs and spirits got scared of me. Khione saw this and said that something had to be done about me. So she sent me here to camp half-blood, where I can destroy this place instead of her... wonderful palace.
Fake: Boreas created Elsa and a couple others since he needed more servants to help him. Elsa was one of his finest servants that Boreas had, but seeing that he only created so little servants, (which he needed more of) he created many more to help him. Many of them mastered many things Elsa and her other friends (the other servants Boreas created) knew and they felt like they had no purpose in life. Boreas saw that and he told them that in the winter, Elsa and the others can help the camp while in spring and summer, they can come back to his palace to help out over there. Seeing it as an opportunity to meet new people, Elsa agreed to go down to camp and help out.
They have the ability to call upon a focused ice storm which freezes a small area into solid ice, the larger the area, the more draining, effects are temporary. They can also use the same ice storm to send small icey shards at an opponent or enemy, the longer they try and sustain the ice storm, the more drained they become
They have the ability to conjure an inanimate object out of pure ice; however, only one can be created at a time and it cannot be bigger than the one who conjured it. They can also use this power to form other objects as well, at delimited size.
They have the ability to create a dome shaped shield of ice, roughly two or three times the size of the user, which can be used to block attacks for a very short time.
They have the ability to turn pure water into ice at will using a cold breeze.
They have the ability to shed their flesh and turn into a state of pure ice
They can camouflage themselves in snow and ice, and can only be seen if they move
They are stronger in the cold
As they are nymphs they do not age, remaining eternally young.
They have a telepathic/empathetic connection with nature and other nymphs
They are able to withstand cold temperatures.
Ice nymphs are seasonal, and would not be at camp during the spring and summer months, they would go where it was colder
More than likely they would be created or turned into one by a god or goddess whose scope encompasses ice or cold