Imagine that the only movie Kim Kardashian ever watched was Mean Girls, and you've pretty much got Celeste. Celeste is totally upper class and effortlessly cool, which presents a striking contrast to our humble heroine.
From the moment we meet her, Celeste makes it clear that nothing will stand between her and Prince Maxon, even if that means she has to play dirty: "Celeste sucked her teeth. 'Please. A girl has more than one way she can pay for what she wants'" (9.8).
We don't think we have to spell out the innuendo on that one. Later, once the competition actually begins, we see Celeste employing a host of dirty tactics, including bullying a girl until that girl flat-out punches her, "accidentally" pouring a drink all over another girl's outfit, and tearing a sleeve off America's dress after America refuses to take it off and give it to Celeste. That's legit insane person stuff.
This conflict grows even more intense once America admits that she digs Maxon. Maxon seems like to Celeste, which seems really weird to America: "How could someone who got along so well with me do the same with someone like her?" (17.35). In other words, how can Maxon be into someone who is the complete opposite of America? We're not sure about the answer, but we have a sneaking suspicion that we'll find one in the later entries of The Selection series.