As we learn throughout the story, Nick clearly has anger management issues. Unable to control his violent impulses, he hits his girlfriend Caitlin, which lands him in court. As well as being violent towards Caitlin, he's also somewhat possessive, which manifests itself in another act of violence, this time toward Dirk at Zack's party.
When some of the kids at the party start trashing the house, Caitlin tells them that they can't do this. Suddenly, the chaos stops, but everyone looks at Caitlin like she's from another planet. One of the kids involved in the mindless destruction, Dirk, approaches Caitlin, stoned and cursing. His eyes are then drawn to Caitlin's dripping swimsuit:
"We're just having fun, baby." He touched her waist. She muttered like a hurt bird. "We could have fun with you, you little—''
But Dirk isn't allowed to finish his sentence, because Nick punches him right in the jaw. He proceeds to punch him repeatedly before Tom lifts him off the hapless Dirk. Zack, whose party this is, calls the police.
As Caitlin tends to Nick's wounds—he has glass splinters in his arms—Nick apologizes to her for what he's done. Yet Caitlin actually tells Nick that he shouldn't apologize and that no one's ever fought for her but him. She calls him her hero, her knight in shining armor. Given how Nick will behave towards Caitlin in due course, these comments are ironic, to say the least.