Rex Smythe-Higgins III is a fictional character in the Hey Arnold! TV Series.
Rex is "Old" Rex's grandson, who acts in a more mature and rational manner than his grandfather. Rex and "Old" Rex have both participated in the Tour de Pond, and against Arnold and Steely Phil. Before the Hey Arnold! series Old Rex and Grandpa Phil competed against each other in the Tour de Pond. Also, Rex with his own grandfather and classmates compete against Arnold with his grandfather and classmates in a re-enactment of the Pig War. Rex has also attended a Polishing School and was on Rhonda's "Cool List". He also has a pet Scottish terrier dog named Pepe who is first mentioned in "The Pig War" and later appears in "Polishing Rhonda."
In the Tour de Pond, Rex does use a comparably advantageous ship in the competition but did not resort to any form of cheating or foul play (like his grandfather) such as rigging another's ship or remote control. Neither does Rex get into a dispute as rashly as his grandfather does to the point of personal attack. Also, Rex isn't competitive to the point he would eat another's pet (such as in The Pig War). Unlike the rivalry between their paternal grandfathers, Arnold and Rex (although they aren't necessarily "friends") appear to be on good terms with each other, as seen in "Tour de Pond" and "The Pig War".