You were Billy's ex-girlfriend, and he was your ex-boyfriend. You and Billy had broken up about 2 weeks ago and at school you see him with Sidney Prescott which makes your blood boil.

Sure, you had some loose screws, but you still loved Billy to death. You were the only person who knew about Billy and Stu's secret. However, would you ever tell anyone? Never. So instead, when Billy is walking home after killing Casey Becker, you hit him in the head causing him to black out. You take him back to your house and tie him in the basement, not willing to let him go.