Professor Thelonious Viper (voiced by Sean Schemmel) is the main antagonist of the first arc of Yu-Gi-Oh! GX season 3. Viper is a professor from West Academy, who came to Duel Academy with 4 other transfer students with a goal: to extract duel energy from the students and give it to Yubel. Viper did all this to raise his son back from the dead. With the help of several others, Viper was close to achieving that goal, until he dueled Jaden and lost. After Yubel grabbed Viper's face, he simply disappeared, which heavily implies his death.
Later on in the Adventure series, Set recruits Viper with the promise to bring back his son if Viper helps eliminate Jaden, Jeffrey, and their adventure team.
Professor Viper uses a Venom deck to show his intentions like that of a poisonous snake. His Main aces are Venominon, The King of Poisonus Snakes, & Venominaga, The Deity of Poisonus Snakes. which specialize in poisoning his opponents.
Trivia:
- In Japan, he is known as Professor Cobra.