writing a book with demons in it?? help?

[Jessica]

the lead female is 17 years old and half demon. shes a demon that rules only when there is no moon in the sky, so once a month. i cant think of any ideas about how to explain HOW she became half demon. or maybe she was born that way? (mother human dad demon or something like that, or maybe other way round) what you think?? in the book she will have been found by her current parents (not a planned adoption) can anyone come up with any ideas that i could use as to HOW she is half demon?? something interesting and maybe a little complicated that will give me a good twist to use in the end. something readers arnt expecting