Justin is the first person that Hannah ever kisses, and he betrays her by taking photos of their interaction without her consent. He later shares them with his friends, leading to Bryce forwarding them to the entire school. Thanks to his troubled home life, Justin often relies on Bryce to give him a safe place to stay. However, Bryce uses this to manipulate him.
At the party, he and Jessica are going to sleep together until he realizes that she's too drunk to consent. He intends to leave her alone, but Bryce forces him to open the door to Jessica's room, leading to her rape. Afterward, he's desperate to keep the information about what happened that night away from his girlfriend. In fact, he's so dedicated that he even threatens Clay with violence to keep the truth from coming out.
Once Jessica discovers that she was in fact raped, and that Justin allowed it to happen, she dumps him. Justin then tells her that he will kill Bryce if she'll take him back, but she's not interested.