In some ways, David is the least rebellious rebel, because he's born into it. David is the young ugly who was born in the Smoke to two runaway doctors; and his main job is to get other uglies from the city to come to the Smoke. But that doesn't mean it's exactly easy.
Sure, Tally and Shay both leave everything they've known behind. They abandon their city, their families, their pretty friends. So when they rebel, they do it hardcore. But David—he's an enemy of the pretty system, sure, but he's born into that position. He doesn't rebel against his city, since his city (er, town) is the Smoke; he doesn't rebel against his parents, since they're also rebels against the system. (In fact, for a rebel, David is pretty polite to his parents: he treats them as equals [30.68].)
But even if David has a loving, non-rebellious relationship to this parents and town, he also gets put into a pretty rebellious position regarding the rest of the world. How do we know he's a rebel? He wears a tough leather jacket, just like James Dean's character in Rebel without a Cause. But we also know that David is a different because that's what we hear all the time. When Shay tells Tally about David, we hear that that name is weird—"It sounded made up, to Tally" (9.21). Trying to describe David, all Shay can say is that "He's...different" (9.30).
So David is the rebel facing a giant system—just like David facing Goliath. And that's especially true after Special Circumstances destroys his town and steals his family. Then he has nothing except his rebellion.
Well, nothing but his rebellion and his feelings for Tally.
Like Shay, David's main role in Uglies is to influence Tally. When Tally comes to the Smoke, David teaches her all about this way of life. He also shows her that even uglies can be cool and comfortable with themselves. For instance,
As always, Tally noticed how graceful David was, how he seemed to know every step of the path intimately. Not even pretties, whose bodies were perfectly balanced, designed for elegance in every kind of clothing, moved with such effortless control. (29.17)
This is an important moment for Tally, since she comes to realize that David is actually better than the pretties in this way. And if ugly David can be better than the pretties, then maybe being pretty isn't all that important. Shock!
And then David seals the deal by telling Tally that she's already beautiful. If being pretty isn't about looking pretty but about being pretty on the inside (oh, what a lovely pair of lungs you have!), then Tally doesn't have to go through the pretty surgery. As David says, "What you do, the way you think, makes you beautiful" (32.101). In this way, David teaches Tally that being (relatively) ugly is okay, and he tells her that she's better than okay.
Or let's put it this way: Shay makes lots of arguments about why it's okay to be ugly; but David actually shows Tally that it's okay to be ugly.