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.
- Because of Tommy's dislike of sports, Donnie seems to be Dwight's favorite out of his 2 sons.
- In Season 1 and 2 he wears a grey t-shirt with a light-blue skull, but in Season 3 and 4 his shirt becomes completely red without the skull (the skull briefly returns in The Old Switcharobot and it's colored black).
- For being a sports fan as well as very strong, he doesn't have any experience in technology such as in "Shelf-Life" it was revealed that he's bad in arcade games and starts to smash one with a baseball bat for losing and that he wanted his coins back. Also, in "Robomonkey Shines" in a flashback, he smashes an old computer of him and throws it out of the window.
- Of all episodes in the series, Donnie is only seen 37 episodes.
- Donnie is absent on the episode "Grow-No-Mo!", though his room and his clothes are seen.
- In Racer Zero, he called Ambassador Mbola "Kingsley". It's been confirmed by Jan that "Kingsley" is the ambassador's first name.
|Dutch||Thijs van Aken (season 3 and 4)|
|Castilian Spanish||Iván Jara|
|Latin Spanish||Eduardo Garza|