Serpentus is a snake-like creature who lives in Castle Grimm, a residence of Castle Gremlyn. The monster is the guardian of the Ultragem, a crystal that turns the holder's thoughts into power. Xenor Krell was an adventurer who set off in search of the Ultragem but was attacked by Serpentus. The act of him crashing through the floor frightened Xenor and caused her to twist her ankle, but Tekno the Canary and Amy Rose appeared, the latter using her crossbow to send him fleeing. Later, when Xenor gets the gem and reveals her true colours, the noie she makes causes Serpentus to return and defend its gem. Unfortunately, the guardian is completely erased from existence by the very gem it sought to protect, his molecules being turned into mist.