Thrax is a deadly virus and a mercenary for hire. Thrax was originally one of Noah Kaiba's assassins, where he explains to Noah how he commits evil, and Noah says he can do it as long as he is okay with it being called "sent to the stars" as it is the same thing as death, which Thrax causes. Thrax is then sent after Naruto Uzumaki as he is the strongest Hero of Legend, making him the most dangerous. Thrax is able to keep up with Naruto, but is ultimately blown off the arena and is saved by Yami Zelda, only for her to let go of Thrax. Despite this, Thrax survives and attacks the heroes again with the Phantom Virus and Sarah Ravencroft. In the second half of Season 16, Thrax, along with Giovanni, is recruited by Ganondorf to help him with his plans. Thrax is also responsible for the deaths of Bowser Jr. and the Koopalings, as he caused them to have a fever that killed them. Thrax is later recruited by Samus Aran, Yami Bakura, and Razanak along with Infinity Mecha Sonic. After the events of Season 17, Thrax is recruited by Zombie Reed as part of the team he's gathered to "spread the gospel". Thrax later helps in the battle against Yami Bakura, his reason being that he hates anyone who breaks his murder record. Thrax decides to stay on Earth with Bowser after Season 18.
- For some reason, Thrax is the same size as a human being.
- Thrax is the only one of Noah's assassins to become a recurring villain.
- Every time Thrax appears, he is under the employ of a different villain, such as Noah, Ganondorf, Yami Bakura, and Zombie Reed.