A lot of character development is not necessarily what it is - though that's obviously important - but how it's given to the reader. You're obviously not going to reveal her entire story in one shot, if you don't start at the beginning.
For instance:
I'm not sure if you're a comic book reader or not, but take Wolverine for instance. When he made his first appearance readers knew absolutely NOTHING about him. (I think it was in an old Hulk issue that he appeared, actually) As he earned his own series and such, they still knew nothing, and it stayed that way for quite some time. Finally, maybe 2 years ago they released his origins. People stayed hooked on him that long, and the still are as more of his apst is revealed. He's beena live for almost 200 years (I estimate). That's a lot of history to present, and spreading it out is the way to do so.
Granted, his history is EXTREMELY fascinating, but it goes along the same lines. As long as you can convince your reader that her motivations are not pure, she has a reason for it.
Since you're relatively new at developing developing characters, as you said, the more questions you can ask yourself, the better off you are. It might seem like a pain at first, but it's useful in development, and also if you happen to hit a snag in the storyline it can be great fuel. Consider these as well, if it helps:
Who are her friends? Who were her friends? What caused them to no longer be her friends?
If she were to go out to relax on a Friday night, where would she go, and who with? What would they do?
Where would she go on vacation?
Who is her role model?
What kind of books does she read?
Is she into drugs? If not, why?
This might also help with the positive traits. Plus, by fleshing her out in this sense, readers might be able to relate to her more easily.