This study simultaneously investigates personal and interpersonal traits that were found to be important factors of bullying behavior using data collected from 1,238 randomly selected Korean middle school students. Using a modified and expanded definition of bullying based on a more culturally sensitive approach to bullying, this study categorizes bullies into three groups: Type I (minor-covert-nonchronic bullying), Type II (moderate-covert-chronic or severe-overt-nonchronic bullying), and Type III (severe-overt-chronic bullying). In addition, this study empirically tests several factors for the first time. Those factors are fun-seeking tendency, teachers' attitude toward bullying, teachers' effectiveness of intervention, teachers' moral authority, power dynamic, and pseudofriendship. The comparison across three groups provided unique findings that different factors were differently related to different groups of bullies. Specifically, teachers have influence on bullying only for the moderate group (Type II), and parents have influence on bullying only for the minor group (Type I). The most important and constant factors across all different groups were prior bullying victimization experience and fun-seeking tendency. (Published Abstract)