Chuck is an unfortunate stereotype: the chubby, lonely kid who has always been a little left out so he just wants to be your friend. If he were a Disney character he'd be Gaston's little sidekick. Or he'd be Chunk from the Goonies, minus the glorious Truffle Shuffle. The second-newest kid in the Glade besides Thomas, Chuck is at the bottom of the barrel in terms of social standing because he's a Slopper, which is the job where you have to clean up after everyone else (imagine cleaning a bathroom being used by 60 teenage boys… that's almost worse than being the janitor at a Taco Bell). All of these things paint a pretty pathetic picture, but that's just perfect because it makes him an appealing little brother figure just in time for Thomas—the Glades newest pariah/hero—to adopt him. If he'd been some kind of Runner super-athlete their relationship wouldn't have the same dynamics, because he'd be someone Thomas would look up to rather than want to shelter and protect. After all, we already have a few of those types with Alby, Minho, and Newt.
Why is this important? Because he's going to save Thomas's life at the end of the book, and the author wants us to weep profusely at his sacrifice. Well, that's half of it. Chuck's death is going to be a major turning point for Thomas, but in order for that to work he needs to have endeared himself to us, the readers. Who doesn't love the poor chubby kid who just aims to please?
The one thing that keeps Chuck from just being a simpering sidekick is his twisted side. On Thomas's first night, when Chuck pulls a prank on Gally and scares the pants off of him, he admits "I love doing this to people. Gives me great pleasure before bedtime" (5.7). Oh, okay, you're one of those sick puppies that derive pleasure from scaring other people witless. Other than this momentary comic relief, though, Chuck's only other dark-side moment comes when he creepily smiles after he tells Thomas about Ben's banishment:
Chuck […] only smiled. Smiled, despite it all, despite the sinister sound of what he'd just announced. Then he turned and ran, maybe to tell someone else the exciting news. (13.44)
Perhaps this glee comes from the thrill of a break in routine, an escape from the monotony of cleaning up after other people day in and day out. Or maybe he takes some secret pleasure in knowing that he's not the worst off in the group for once. Either way we are given a tiny glimpse into the fact that with Chuck, there is more than meets the eye.