Will's our favorite type of character. No matter what the situation, he'll try to make a joke, even if most of his jokes aren't funny or nice. Will is originally from Erudite, and that upbringing comes out in some little ways. For instance, he memorized a map of the city for fun and seems a little confused that Christina didn't (11.41). Plus, listen to him talk about how interesting the fear serum is: "It's basically a struggle between your thalamus, which is producing the fear, and your frontal lobe, which makes decisions. But the simulation is all in your head, so even […]" (19.38)—and he goes on and on.
But like Christina, Will isn't just defined by his original faction. (If he were, he probably would've stayed there.) His major quality is probably his mischievous, joke-telling nature. His first line to Tris emphasizes this: when Tris is having trouble hitting a target, Will says that she should have hit the target by now, at least "Statistically speaking" (8.15).
But not all of his jokes are so Erudite (read: nerdy), and many of them lean towards the dirty. He makes a joke about Tris being sexually frigid because she doesn't like public displays of affection (8.47). Then he makes a joke about Tris getting a nipple pierced (8.77), which is all kinds of awkward. And all of this is in one chapter. The correct response to most of Will's jokes is to groan or sigh, as Tris does (8.78).
Nevertheless, Will's a good guy to have around. For one thing, he's a good friend because he shares and has Tris's back. For example, after Edward gets stabbed in the eye, Tris and Will go hang out alone and drown their worries in cake, which just goes to show you how similar Tris's life is to ours today (16.140). He's there with Tris and Christina as they symbolically destroy some Erudite articles badmouthing the Abnegation government (25.1). And when Al tries to apologize to Tris for nearly killing her, Will is there: "Will stands behind me and holds my shoulders—lightly, as if to remind me that he's there" (23.86). Which is about the most literal example we can find of someone "having Tris's back."
Which is why it's so sad that Will succumbs to the sleepwalking mind-control of the Erudite; and that when he's trying to hunt Tris, she's forced to shoot him in the head. Or is she? Awkward.