Dr. Rex (first name unrevealed) was a well-meaning physician who set the fugitive Pruneface's broken leg. He was bald and had a dark moustache. He had thick eyebrows and wore eyeglasses.
Dr. Rex grew suspicious when he found the cloth disguise Pruneface was wearing was his own dog's sweater. He deduced that Pruneface must have harmed the dog in order to disguise his wrinkled features.
Pruneface drew a gun and forced the doctor to call his confederate Agent 20 and an ambulance. After loading the ambulance with Pruneface, Agent 20 hijacked the ambulance and drove Pruneface and Dr. Rex to his home. Rex was able to leave a clue of the house number in the ambulance where it was found by Dick Tracy.
While Pruneface was recuperating, Dr. Rex attempted to attack him with a hat stand. Pruneface shot the hat stand and expressed amusement at Rex's defiance. After Pruneface killed Agent 20, he allowed Dr. Rex to leave the hideout with Agent 20's corpse as a warning to the police..