Cool. So you put a lot of thought into her. I respect that. Do you have a post detailing this background in your gallery? I was trying to look through the whole thing, but you're quite prolific. 'Course, that's a good thing. If you can link me to her character info, I'd be interested in reading it.
I don't think I have that information posted anywhere, but the long story short is kinda like this:
First, she's not an avatar of any goddess, so you can count that out. Nyx's real name is Nikeiva Kasta, and she's my superheroine character. She was not born with powers, nor with her pointy ears-- they were the result of an experimental drug she took to treat an otherwise terminal illness. While she is cured of said illness, the drug's after effects didn't stop with bodily mutation and powers... it changed her psychological state, and either caused her to develop a second personality.
The second personality is dark, violent, and cold, and she took out her aggression by killing the doctor who helped her. (Before you grieve for him, he was a slimeball and was asking for it.) Traumatized by what had happened to her, she went through a period where she was switching personalities rapidly, and was very unstable. She likens the times when the darker personality takes her over to night falling, and bringing with it all manner of scary things she can't control. Being of roughly just-past-college age, she recalled a mythology course she'd taken, and called the other personality 'Nyx', as her way of describing her. She both hates and fears this other side of herself, but at the same time knows that it is somehow essential to her, because it holds strength that she can't or won't tap into when she's in her right mind.