Guardian of the Labyrinth, known as Guardian of the Underworld in the Japanese version, is a character version of the card "Guardian of the Labyrinth", who appears as part of The Supreme King's army in the alternate dimension in Yu-Gi-Oh! GX.
Guardian of the Labrinyth appears alongside Skull Knight while the latter is disguised as "Ancient Brain". The two infiltrate the final free town in the alternate dimension. His shield does most of the talking, with the monster itself only responding to what the shield says by saying "exactly". Skull Knight plans to report back to the Supreme King, telling Guardian of the Labyrinth to throw open the gates of the village when the King's forces arrive. Axel Brodie confronts them and Skull Knight reveals his true form, challenging Axel to a Duel.
When Skull Knight is defeated, Axel captures both Guardian of the Labyrinth and its talking shield. What happens to the Guardian itself after this is unclear, but Axel takes the shield hostage to use a guide while infiltrating The Supreme King's castle. He, Zane Truesdale and Aster Phoenix disguise themselves in the armor of "Dark Blade" and infiltrate the castle. They are confronted by Chaos Sorcerer, who questions what they are doing. The shield responds that they have a message for the Supreme King from Skull Knight. Chaos Sorcerer pretends to let them go, but then attacks, revealing that all messages from Skull Knight are to go through him prior to reaching the King. The shield reveals their true identities, telling Chaos Sorcerer that the three are here to confront the Supreme King. Despite this, Chaos Sorcerer kills the shield for its treachery.