Skull Guardian is a character version of the card "Skull Guardian", appearing in Yu-Gi-Oh! Capsule Monsters.
Skull Guardian was the creature of darkness in the second of the Trials of the Capsule Monsters, in the World of Capsule Monsters. He appeared after the creature of light, Prisman was defeated.
Skull Guardian emerged from the remains of "unbreakable stone", after Joey Wheeler, Tristan Taylor, Solomon Muto, Flame Swordsman and Thunder Kid broke it and defeated Prisman during the Trial of Light and Shadows. They were surprised to see Skull Guardian, believing they had completed the trial. However they were unaware of the second half of the trial's clue; When light is extinguished, the true test begins for a creature of darkness it brings. But only the righteous and peaceful shall pass and receive the endowment of wings.
Skull Guardian was able to deflect attacks from Flame Swordsman and Thunder Kid. Yami Yugi, who had read the complete riddle arrived to protect his friends from the "creature of darkness", Skull Guardian. He summoned Dark Magician, fused him with his Duel Armor and proceeded to fight off Skull Guardian. Once Skull Guardian was subdued, a treasure chest appeared in the remains of the unbreakable stone. Joey ordered Flame Swordsman to finish Skull Guardian off, but Yami blocked the attack. Per the riddle, they needed to demonstrate that they were righteous and peaceful, so he showed mercy to Skull Guardian.