Dox, known as Kyū in the Japanese version, is the younger of the two Paradox Brothers. He and his brother Para Duel as a tag team, and appear to Duel the protagonists in both Yu-Gi-Oh! and Yu-Gi-Oh! GX. Dox has the kanji for half of the word "Labyrinth" on his head, with his brother's head having the other half.
He and his brother were hired by Maximillion Pegasus to work as Eliminators in his Duelist Kingdom tournament. The two duel as a team, with their "Labyrinth Wall" card enabling them to change the playing field to resemble a complicated underground labyrinth, making gameplay more akin to a board game than a card game. The two guard an underground area of the island. It is here that they meet Yugi Muto and Joey Wheeler, who must defeat them in order to pass and move onto Pegasus' castle. They begin a Tag-Team Duel against one another, and the Brothers have a cohesive strategy, with their Decks built to play off of each other's. In addition to changing the gameplay, the brothers' Decks focus on the Summoning of their powerful "Gate Guardian" monster, by gathering its component pieces onto the field.
Though they come close to victory, the brothers are defeated when Yugi fuses his "Summoned Skull" with Joey's "Red-Eyes Black Dragon" with "Polymerization" to form "Black Skull Dragon", whose ATK was able to surpass that of their "Gate Guardian" when Joey uses his "Copycat" card to use the brother's own "Riryoku" against them. With their victory, Yugi and Joey both obtain enough Star Chips to gain entry into the tournament's final rounds. However, the Brothers still have another test that Yugi and Joey must pass. They inform them that one of the doors that leads out of the room will let them leave the caves, while the other simply leads to an endless maze. The brothers claim that from here on out, one of them will speak only lies, while the other will speak only the truth. Yugi realizes that both brothers are in fact lying, and they even have the ability to change which path leads where. They are tricked into admitting this via the use of a two-headed coin, and Yugi and Joey are able to move on.
The Brothers are hired by Dr. Crowler to Duel Jaden Yuki and Syrus Truesdale, who will be expelled should they lose, due to their trespassing at the Abandoned Dorm. The Duel is conducted in a more normal fashion this time, with the two Dueling using standard Duel Disks. Their strategy still involves the Summoning of "Gate Guardian", but they also reveal an even more powerful force - "Dark Guardian", who can only be Summoned upon the destruction of "Gate Guardian". The brothers focus their efforts on Syrus, believing him to the weak link of the opposing team. This is very successful, but Jaden eventually gives Syrus the confidence to continue, and the brothers are once again defeated when their opponents pool their resources to Summon a Fusion Monster. This time, Syrus uses his "Power Bond" card to fuse his "UFOroid" with Jaden's "Elemental Hero Tempest", forming "UFOroid Fighter", making its 4000 ATK points double to 8000, which was enough to attack "Dark Guardian" and wipe out the brothers' Life Points, even though it cannot be destroyed by battle.
In Yu-Gi-Oh! GX Tag Force, the Paradox Brothers participate in the Tag Force tournament. After completing the game once, the player may partner with either of them, and they can be found during part one of the game on the cliff. Upon acquiring his Deck Recipe, Dox also gifts the player with a "Labyrinth Wall" card.