Neville is a fellow Gryffindor student, and largely seems pretty timid and tame. He's well liked, but even his friends tend to think of him as kind of…well, kind of a weenie.
Case in point: with the Yule Ball approaching, Ron just assumes that (like Harry and him) Neville hasn't found a date yet:
HERMIONE: It might interest you to know that Neville's already got someone.
RON: Now I'm really depressed.
Given that rep, it's kind of a shock when we learn that Neville's father, Frank Longbottom, was an Auror (translation: a super-brave dark wizard hunter). As it turns out, Neville's parents were both tortured at the hands of Barty Crouch Junior (a follower of Lord V, naturally).
We even find out (unfortunately) a fair bit about the agony Neville's parents were forced to endure when the Hogwarts students get a tutorial in Unforgiveable Curses in their Defense Against the Dark Arts class. The class' professor, Mad-Eye Moody (who's actually Barty Crouch Junior), asks Neville to name one of those spells, and he comes up with (as we learn later) the curse that Barty used to punish his anti-Death Eater parents:
MOODY/CROUCH: Up, up. Come on. Longbottom, is it? Up. Professor Sprout tells me you have an aptitude for herbology.
NEVILLE: There's the, um…the Cruciatus Curse.
Then, Moody/Crouch demonstrates the spell on a poor spider, right in front of Neville:
MOODY/CROUCH: Correct, correct! Come, come! Particularly nasty. The torture curse. Crucio! [performs the spell]
HERMIONE: Stop it! Can't you see it's bothering him? Stop it!
Moody/Crouch can see that it's bothering Neville, as it turns out. In fact, he's purposely showing Neville the curse that he himself used on the boy's parents. No wonder Neville reacts like that.
Moody/Crouch does make a solid observation here, though: Neville is good at herbology. Unfortunately, Moody/Crouch uses Neville's interest in the subject to interfere with the Triwizard Tournament. He feeds Neville information about Gillyweed, which Neville then relays to Harry so he can succeed at the tournament's second task.
(Sure, it's good that Harry survives the task, but "surviving" only brings him closer to the confrontation with Voldemort…so, not actually a good thing.)
Bottom line: Neville is a good kid, but he's had kind of a rough time, and Moody/Crouch doesn't help matters at all.