What do you need to know about Greg? He's a jerk.
But he wasn't always that way. In elementary school, he tries to defend Jin when Timmy bullies Jin for being Chinese, telling Timmy, "'Hey, be cool, man'" (2.43), and ends up redirecting Timmy's anger toward him.
That's as far as the defense goes though, because once Timmy makes nice with Greg and says, "'Come on. Let's leave bucktooth alone so he can enjoy Lassie'" (2.47), Greg follows Timmy and doesn't say anything else to defend Jin from Timmy's clearly racist dig.
Even though we don't see Greg that much in the book, we can kind of see how elementary-school Greg becomes his junior high self. Even though he has the right idea in elementary school to defend the underdog, he still cares about fitting in too.
So when junior high rolls around, Greg—who's clearly your stereotypical version of a cute, blond, popular guy by virtue of the way Yang draws him—isn't a defender of underdogs anymore. He's at the top of the junior high heap and wants things to stay that way (especially with the girls).
How so? Greg never really becomes a big deal in the book until he happens to witness Jin and Amelia out on their date, all while he happens to be on a date himself with some other girl (8.42).
At this point, Greg becomes the antagonist in Jin's world because he treats Jin in the most damaging way possible; he tells Jin that Jin—essentially—isn't good enough for Amelia:
"It's just that she's a good friend and I want to make sure she makes good choices, you know? We're almost in high school. She has to start paying attention to who she hangs out with." (8.62)
Not cool. What's worse, Greg tries to make himself into a nice guy. He tells Jin, "'Aw, geez. Look, Jin. I'm sorry. That sounded way harsher than I meant it to. I just don't know if you're right for her, okay? That's all'" (8.64). Does anyone actually think he's really sorry for barring Jin from dating Amelia? To be clear, we definitely aren't buying it.
Greg's the kind of bully who's even worse than Timmy; he's a bully who tries to act like your best friend. Ick—a classic case of a frenemy if you ask us.