A big part of the game is playing off of stereotypes that are common to VNs.
Maeve was created first and is intended to be the archetypal Pretty Girl. She wears ridiculously sexy clothing and loves the color pink. You could easily see her as the captain of the cheerleading squad and the prom queen. So I liked the idea of taking the cheerleader/prom queen and giving her this aloof, kind of nerdy personality that can be frustrating and annoying. Kind of a yin/yang thing: on the one side she's the perfect Pretty Girl, but on the other side she's fucking weird.
Shannon was created largely to be the anti-Maeve. While Maeve can be annoyingly indirect, Shannon can be obnoxiously direct. Where Maeve is like a butterfly flying around your head, Shannon is like a gut punch. From there it's just a matter of adding layers.
For Olivia, I knew I wanted the stereotypical mother + two daughters setting. And I wanted things to be somewhat realistic. So the only somewhat realistic way to have a mother who's possibly sexually interested in this young guy entering her house is to make her totally irresponsible.
I've said before, but Angel is simply my goth dream girl.