Derek Dietl is Susan's ex-fiancee and the secondary antagonist in the movie Monsters vs. Aliens. He and Susan were supposed to get married in the beginning of the film, but Derek doesn't look forward to get married and spend the rest of his life in somebody's shadow.
Derek starts off handsome and is very charismatic, but he really is a selfish arrogant jerk. When he tells Susan that they're not going to Paris, he says that they're going to Fresno, only so he can take the job interview. He's a narcissist who only cares so much about his looks and his career which hurts Susan really bad.