The DUFF discussion

In "The Duff" by Kody Keplinger, Bianca and Casey are foil characters for each other. A foil character is a character that highlights another characters qualities through contrast.
Bianca and Casey are opposites. Bianca doesn't like going out to parties and is not into drinking. Casey, on the other hand, loves doing those things. Bianca's traits accentuate Casey's traits through their contrasting qualities.

Your right I fairly enjoyed this book, it was quite funny. I will have to be honest to say that I liked the movie better.