Donnie Turnbull is Tommy's older brother (and later, baby brother but only at the end of the episode "Zap! You're Old") and is a secondary antagonist in the series, who is a constant bully to almost everyone he encounters, including Gus, and Tommy himself. He frequently calls people names, threatens them and yet despite this Tommy still cares about him and treats him as family. Sometimes he has been exploited due to his attitude for other people's benefit, particularly by Dr. Kamikazi.