Jenna is the sister of Jonathan (Joey's roommate), and the object of Joey's desires while in college. Even while he's together with Connie (and married to Connie), he's secretly speaking to Jenna on the phone daily, in hopes that she might eventually let him kiss her.
What's Jenna like, you ask? Here's what Joey thinks:
Jenna excited him the way large sums of money did, the way the delicious abdication of social responsibility and embrace of excessive resource consumption did. He knew perfectly well that Jenna was bad news. Indeed, what excited him was wondering if he might become bad enough news himself to get her. [...] She was even worse news than her brother had made her out to be. (3.5.56)
What are we supposed to make of this girl? If Joey is the embodiment of America's worst instincts, then what is Jenna? Why, she's the object of those desires, of course. From Franzen's perspective it seems that she's the coal lying underneath the pristine mountaintop; the profitable privatization of prisons; the sub-prime mortgage gambled on Wall Street at Main Street's expense. And, sure, those things might be tempting – "Ooh, imagine that? That would be so sweet" – but indulging in those desires, as Joey learns, just makes a big smelly mess of everything.