Diomedes is a Greek soldier. After escorting Cressida to the Greek camp, he puts the moves on her and convinces her to sleep with him, despite the fact that she promised not to cheat on Troilus. When it comes to romance, Diomedes is the exact opposite of Troilus. He's not interested in playing any of Cressida's games and when she plays hard to get, he's prepared to walk away: "No, no, good night, I'll be your fool no more" (5.2.32).
But then, when Cressida gives in, Diomedes throws it in Troilus' face on the battlefield by wearing on his helmet a love token that Troilus once gave Cressida. Yep. That's the Shakespearean equivalent of bragging in the locker room about stealing another guy's girlfriend.
We don't know, Shmoopers—this guy is obviously no good, but we can't help liking him a little. He doesn't take any nonsense, you know? Or maybe we just have a thing for the bad boys.