Avaric is the classic spoiled rich boy, straight out of an '80s teen movie. Even worse is that he actually stays good looking and successful later in life. Yeah, we know.
So what's he doing in this novel? Is he there for contrast's sake?
Maybe. But Avaric also shows how some people just luck into a charmed life. Avaric is also kind of a male Elphaba, at least in terms of sarcasm. They end up having an excellent bantering, frenemy-style relationship in their adulthood. Avaric kindly invites Elphaba over for dinner and a conversation about evil after she tracks him down to "confess to" (lie about) Madame Morrible's murder.