Mr. Rex is, like his daughter, a tyrannosaurus rex. However, while Tina is shown to be taller than almost students at Elmore Junior High, Mr. Rex is shown to be taller than most people in the city of Elmore; towering above Gumball, Nicole, and even his own daughter.
Like Tina, he has thick, brownish-green scales all over his body and huge, black claws on his feet and presumably his hands as well. He has an enormous mouth filled with rows of razor-sharp teeth, and piercing, red and yellow eyes.
Mr. Rex is extremely fierce and protective of his daughter and his home. Unlike his daughter, he is shown to be very animalistic and brutal, preferring to settle disputes by fighting instead of talking.
- Like Hector, it is impossible to see Mr. Rex in the normal view of the camera since he is so enormously tall.