You could argue that the real important division in this book isn't between individual characters, but between different groups of characters: the uglies (who haven't yet been operated on), the pretties (who have been operated on), and the Specials (the law enforcers).
Ready for a look into the Shmoop brain? Our real favorite characters are Sussy, An, and Dex, for one simple reason: no one remembers who they are, so it makes a devilishly hard quiz question.
Fine, we'll tell you: Sussy, An, and Dex are three uglies who help out the Smoke by pulling off (and getting other tricky uglies to pull off) a big distraction during the break-in at Special Circumstances. Almost all the older uglies we see in this book are trick-pulling pranksters, whether they know about the Smoke or not.
We might even say that tricks define uglies; for instance, when the young uglies move in to the dorm and Tally is almost the last old ugly, she teaches them all her old tricks (11.5). As Tally tells Shay, "when you're pretty you might not need to play tricks and mess things up" (10.80). In other words, she'd better do it now.
And then there are the new pretties, like Peris, who spend their days at parties instead of pulling tricks. For instance, when we see post-surgery Shay, she's just come from a party—but not her first one: "This is not my first party tonight, you know" (46.25). Sure, they may be having fun (how can you not have fun with "safety fireworks" and bungee jackets?), but they don't seem so nice.
When Tally crashes a fancy dress party with her pig mask, she sees,"Nothing but surprised looks and pointed fingers, and pretty faces" (2.22). Pointing at the weird-looking kid is a sure sign of jerkhood. And then the pretties hunt her down while yelling "Piggy." And that is never a good sign.
So that's the big breakdown between these two types of characters: uglies spend their time pulling tricks (breaking the system in some way) while pretties spend their time partying (within the system) and being mean to kids who don't fit in. Boy, the future is totally unlike the present, right?
Which brings us to the Specials. The Specials are a special case (oh, we hate ourselves for saying that). We see this pretty clearly during the invasion of the Smoke when we find out that"they were superhumanly fast and strong. The Special operation had given them more than just terrible faces" (33.9). We don't get to see a lot of the Specials; Dr. Cable is the Special we spend the most time with and her Specialness basically comes out in how scary and mean she is (see her "Character Analysis" for more about that).
If you usually want to help pretties (because they look so vulnerable), Specials are the kind of people you want to avoid or run away from: "cold, commanding, intimidating, like some regal animal of prey" (12.42). Even status quo Sol and Ellie now how scary-sounding the Specials are: "you wouldn't want to meet" a special, says Sol (14.48). That's how scary the Specials are: even law-abiding pretties don't want to get tangled up with them.
So uglies pull tricks (and so fight the system, though often in ways that are approved); new pretties party all the time and then get jobs (and so go along with the system); and Specials protect the system by finding troublemakers who would threaten the way things are (and scaring people who aren't rebels). Yep, the future is totally different.