Rex has pale skin, blonde hair and blue eyes. He wears a blue, long-sleeved jacket over a white T-shirt with brown pants. He also wears blue sneakers and owns a dinosaur tooth necklace, similar to that of the other members, except his is larger than Zoe's and Max's.
Rex is the quieter one, with a mysterious past that he occasionally thinks about. He was adopted by paleontologist Dr. Owen, Dr. Taylor's friend. He had been born in the Mesozoic Era to Dr. Ancient and Dr. Cretacia during their expedition on the Backlander. He has fought with Max on several occasions, much to Zoe's frustration.
In the Taiwanese game, it notes that Rex works well with Max. He analyzes fossils found by Max, and that he is better with dinosaurs than Max.
Rex was the only child born to Dr. Ancient and Dr. Cretacia. After Rex was born, his parents began to fight with Dr. Z, who made dinosaurs fight for his amusement. Later, Dr. Z teamed up with Seth to get rid of them. When they put their plan into motion, baby Rex was sent to the present, while his parents were sent flying somewhere. He was found in the Smithsonian Museum by a bachelor named Dr. Owen, who adopted him. Since he was a traveling paleontologist, it interfered with taking care of Rex. Thus, he sent him to his close friend, Spike.
Many years later, when he and the D-Team are twelve years old, Max sees a meteor falling from the sky, and wakes up Rex. The next morning, he, Max, and Zoe travel to the forest, where they find three stones with Wind, Lightning and Grass on them. Rex picks up the Wind stone. Then, Max finds two cards: a Triceratops and another odd card. He rubs the Triceratops on his Lightning stone, where it comes to life! However, Max soon discovers ho to transform it back to a card and eventually, how to create a miniature, chibi version of the Triceratops (who Max names Chomp).
Later, D-Team had to face the Alpha Gang, who summoned Terry the Tyrannosaurus. Rex notes that it would be impossible for a Triceratops to defeat a Tyrannosaurus, however, Max and Chomp defeat it with the move card which Max found: Electric Charge.
Later, Dr. Owen finds two more cards, a Carnotaurus (with attribute of Wind) and a Parasaurolophus (with attribute of Grass), named Ace and Paris by Rex and Zoe respectively. D-Team gets along well with their dinosaurs, often keeping them in their chibi forms (In the chibi form, Max's mom thinks of Chomp, Ace and Paris as dogs). They constantly fought the Alpha Gang who wanted all the dinosaur cards and move cards, with Rex and Ace often fighting Tank the Saichania.
Later, Rex and D-Team were told the truth of Rex's parents through Jonathan: Rex was born to Dr. Ancient and Dr. Cretacia in 2126 A.D.
Dinosaur King: Mesozoic Meltdown
Rex met his parents, who came back. Rex's parents take the dinosaur cards and the element stones from D-Team and Alpha Gang. Rex, his parents, Jonathan, and Alpha Gang fly back to future through Backlander. However, due to Spectral Space Pirates' attack, they had to come back. Gavro later kidnaps D-Team's parents.
They fly from one time period to another, leaded by Pterosaur. They eventually begin to find Cosmos Stones. After the defeat of Spectral Pirates and Seth, Rex, his parents, Jonathan, and Alpha Gang fly to the future. Because the Backlander was damaged, they used the spaceship of the Spectral Pirates, which could take only one flight (it also meant that Rex could not come back from future to meet Max and Zoe ever again).
Rex's dinosaur is Ace the Carnotaurus, a Wind attributed dinosaur. Ace is the fastest member of the team, as well as the only carnivorous dinosaur. Ace, in his chibi form, is very playful and occasionally gets on Rex's nerves, but Rex can't imagine life without his little buddy. In full form, Ace is a powerful dinosaur to fight against, and his agility can dodge moves that come his way.
'Carnotaurus, blow them away!' (Summoning Speech)
In the manga, Rex is an American transfer student. He at first didn't speak Japanese and could only speak English, so no one understood him. Although one day he was reading a book and Max jumped out and scared him, and then notices Rex was reading a book on dinosaurs. Max says he loves dinosaurs too, and he wants to be a paleontologist when he grows up. Max then remembered Rex didn't speak Japanese, so he showed with gestures and hand signals that he loves dinosaurs. Soon he and Rex become best friends.
They one time find a fossil together, but it breaks, so they share the two pieces; Max puts the small bit on his waistpack, while Rex wears the bigger piece around his neck. Rex also is very smart and is seen often blowing pink bubblegum, and a lot of the girls in his class like him. Rex has also mentioned having parents and also mentions to Max that he owes him a certain number of puddings for doing something dangerous or something he doesn't want to do with him.
His dinosaur is Benjamin, a carnivorous dinosaur that was found by Max at Alpha Mountain.
The player can choose to play as Rex, and they start out with Carnotaurus.
- Rex is the only character with a given birthday, which is on June 10.
- In Kyoryu-King.com's 4komas, Rex is occasionally shown with a form of anthropomorphism as a half human, half Carnotaurus person. In this state, he can use wind move cards.
- He is one of the few characters to keep their original name in the dub, as his name is already English. Ace also kept his original name in the dub. Other characters are Spartacus, and Aki Taylor (Max's mother) kept her first name the same, although her last name changed.
- Although his real name is Rex Ancient, the Japanese website keeps his name as Rex Owen, which may mean that Rex chose not to adopt his real family name.