She's one of the popular girls, but not the main one. She is one of the groupies who hands off to the side. She acts highly superior, girly, and silly in school. She's also not the brightest grade-wise.
However, out of school, she's actually quite smart. It's all an act, it turns out, but your character may or may not know this. She's highly calculating and always planning to take down the most popular girl. 
Is she a killer-to-be?Does she know something she isn't supposed to?Perhaps undercover?