Character Clues
Character Analysis
Actions
We learn a lot about Marcus and the other characters through what they do, even before they start their plan to bring down the DHS. When Marcus's called to the vice principal's office he has to take care of his laptop's multiple downloads, then leaves his social studies classroom with his "arms over [his] head like a prizefighter" (1.8).
We get the idea that this guy goes to the office a lot—he doesn't seem too stressed out about it.
Marcus later shows the importance of actions by ignoring the loudspeaker telling him and Van to stop talking in the prison yard (4.46-55). How Marcus reacts to authorities tells us that he sticks to his convictions, cares about doing what's right more than just blindly following the rules, and that he's probably just a teensy bit full of himself.
Direct Characterization
As the narrator, Marcus tells it like he sees it…and makes sure we see it like that too. Take the first time we meet Charles; Marcus tells you what you need to know:
Charles Walker and I don't get along. We're in the same grade, and we've known each other as long as I've known Darryl, but that's where the resemblance ends. Charles has always been big for his age, and now that he's playing football and on the juice, he's even bigger. He's got anger management problems — I lost a milk tooth to him in the third grade — and he's managed to keep from getting in trouble over them by becoming the most active snitch in school.
It's a bad combination, a bully who also snitches, taking great pleasure in going to the teachers with whatever infractions he's found. Benson loved Charles. (1.23-24)
Charles's a bully, a snitch, and has anger management issues. So, we're guessing that he's not a super fun guy to hang out with.
Physical Appearances
Characters don't just get defined through their actions and direct description. What people look like (especially law enforcement agents) signals who they are. Take "severe haircut lady" (3.99). She's grim, crisp, and shows no mercy. It's no surprise then that she locks Marcus up and eventually has him waterboarded.
Others defined by physical appearance are the cops Zit and Booger (6.87-7.5). Marcus never calls these people these things out loud, just in his narration. Since most of these characters never get named (except for severe haircut lady, but only at the end), physical appearances are another way that Marcus distances us from law enforcement officials.
His friends and other characters get some physical description, but Marcus talks more about their personalities than their looks.