According to the compensation model of aggression (Staub, 1989), some people bully to defend against their own feelings of weakness and vulnerability. Classmates and teachers rated a sample of American sixth graders in terms of trait: defensiveness (i.e., defensive egotism), self-esteem, bullying, and related behaviors. Consistent with the model, students' peer- and teacher-rated defensive egotism were positively associated with bullying and physical and social aggression, respectively. Students' peer- and teacher-rated self-esteem was negatively correlated with bully victimization and positively correlated with defending victims. Some findings were qualified by regression analyses using personality, gender, and their interaction as predictors of focal behaviors. The positive association between defensive egotism and bullying held only for boys; yet, at higher defensive egotism, girls were more socially aggressive than boys. The findings, along with previous research, suggest that bullying-reduction programs in schools include a component on the personality and motivational dynamics of bullies, victims, and victim defenders.