Rex Raptor, known as Dinosaur Ryuzaki, in the manga and Japanese anime, is a "Dinosaur" duelist. Despite his introduction as a powerful duelist, he becomes a source of humiliation along with his companion Weevil Underwood after suffering defeat to Joey Wheeler and Yugi Muto. Rex develops into a rival of Joey. Rex is vengeful, arrogant and tyrannical. Yet, he is confident and plays fairly.
"Dinosaur" is a nickname; "Ryuzaki" is his given name. In the English anime, he is named after the Tyrannosaurus Rex and the Velociraptor, 2 types of dinosaurs.
In the Japanese anime, Ryuzaki speaks with a Yokohama accent.
Rex is invited to the Duelist Kingdom tournament. Being a runner-up, in the last tournament, he is one of the few participants given a room on the boat journey. He invites Mai Valentine to his room. Mai challenges him to a Duel, where she'll give him a kiss if he wins (Do anything he wants for him the Japanese Version), but she'll get the room to herself if she wins.
Confident that he won't lose, Rex accepts. However Mai insists that they use her cards. Unbeknownst to Rex, she's sprayed them with different perfumes and is able to tells what cards he draws. With this advantage, Mai wins and boots him out of the room.
During the tournament, he is constantly berated by the fact Mai has more Star Chips than him. He challenges her to a Duel to get revenge but Mai, preferring to avoid him was content to use the excuse that she had already "beaten" him once. However since Mai had previously lost to Joey Wheeler and considered it obnoxious to challenge him again, she agreed to duel Rex, if he defeated Joey for her.
In the Waking the Dragons arc, it is revealed that he and Weevil lost a good deal of their popularity after the tournament and they eventually ended up wandering Domino City where they were ambushed by Grerimo. Their souls are taken and released when Yugi Muto defeated him.
The two of them then trail after the party until Yugi's soul is taken by Rafael and they beg him to take them on their plane. Rafael and Alister take them to Dartz, who agrees to give them power if they pass the necessary test. Each of them is given a piece of the Orichalcos Stone and both must succeed in grasping them, which both of them do.
They then improve their Decks by taking cards from Dartz's massive collection and Rex challenges Joey to a rematch. Despite using "The Seal of Orichalcos" Rex loses, sending his soul to Dartz again. He is released at the end of the arc when Dartz is defeated. He and Weevil flee from the hospital they found themselves in.
Rex and Weevil's last attempt at revenge was to steal Yugi's Egyptian God Cards from his house, where they mistakenly grab Yugi's bag containing all of his Millennium Items. Yugi tried to stop them but couldn't catch up. While examining their prize, they were then attacked by Yami Bakura, who stole the Millennium Ring back from them.
Ryuzaki is shown in the last episode of Yu-Gi-Oh! dueling with Haga.