Unfortunately, in a book with such a crazy-high death toll, we don't get to know much about the minor characters… because they all die within minutes of us meeting them.
Darnell is just, well, this:
A teenage boy appeared at the door with a pair of underwear pulled over his brown hair like a hat. Darnell. (2.31)
He's… a dude who wears dirty underwear on his head. And he also gets shot with a dart in the very beginning and dies a few days later.