Her eyes said it all—they were going to live though this just like they'd lived through everything else. (12.35)
The eyes say a lot of things; Trina's characterization can be defined pretty early on just by Mark's description of her here. She's one tough cookie.
Mark knew they'd become hardened […] But this was the first time they were faced with leaving someone who still seemed so alive. (13.31)
It's one thing to have to try to survive a natural disaster and an assault from people on a Berg, but it's another thing to leave a friend behind to die. It actually takes courage to do this—and it takes courage to stay behind, too.
"You won't touch her," Mark says, the look in the man's eyes filling him with the bravery he couldn't find a few minutes earlier. (14.26)
Mark is pretty protective of Trina, but he isn't the bravest kid during the sun-flares disaster. When someone threatens Trina, he suddenly gets a burst of bravery—hmm, wonder why.
He refuses to believe his family is gone and swears to himself that he won't rest until he finds them safe and sound. (19.15)
Sure, it might be denial to refuse to admit that your family is dead, but it's also courageous of Mark to think that he can save them.
Almost seventeen years of life, and he never knew what a coward he was. (20.7)
Please, Mark. You're sixteen years old and the sun is melting everyone around you… so don't think you're a coward just because you get scared every once in a while.
Maybe, just maybe there was a spark of hope. Maybe. How many times had he faced death in the last year or so? He was used to it by now. (44.3)
Hope is something that's rarely found in Mark's world, so it's pretty courageous of him to even try to find a spark of it. Especially because this is when he knows for sure that he has the Flare.
The task ahead of them was treacherous and scary. But for some reason his mind refused to focus on that. (48.31)
This is the kind of person you want as the hero: someone who isn't going to let their nerves get the best of them.
"I'll be fine," she said, cutting him off. A sudden fire burned in her eyes that made her seem ten years older. "Let's go." (57.21)
Speaking about courage, how about that Deedee? For a five-year-old kid, she has tremendous courage. Not only does she have to put her trust in complete strangers, but she's actually given some pretty large tasks and accepts them without complaining.
"Let's go up, then. Come on, it's going to be okay." He'd never said anything in his life that he believed less. (58.10)
Sometimes lying out of your teeth to make someone feel better is the hardest thing to do. It takes a certain amount of courage to allow yourself to say things like this.
But instead she looked him in the eye and nodded. Mark's head wasn't clear enough to understand how she could be so brave. She was special. (67.24)
Well, yeah. She turns out to be Teresa—one of the bravest Gladers we see. Thomas often hints at how Teresa is particularly brave, but now we know why.