Marius is the guardian of the Avancia Labyrinth, tasked with its defense through combat and management. He appears as a humanoid with long white hair tied in a ponytail, blue eyes, and typically wears a long-sleeved kimono. This form masks his ability to transform into a giant skeleton, revealing a monstrous aspect during battle. Marius wields two katanas in combat and thrives on challenging fights, taking great pleasure in facing formidable opponents.

Initially encountered as an adversary within the labyrinth, Marius battles Rud but is defeated. Rud spares him, leading to an unexpected friendship. This pivotal event makes Marius the first sentient labyrinth guardian to form an alliance with Rud. Following his defeat, Marius provides Rud with his first labyrinth treasure and becomes a recurring ally. He accompanies Rud on subsequent labyrinth raids, offering tactical support and sharing his expertise in labyrinth operations. Marius demonstrates practical techniques like monster placement, environmental layout alterations, and general dungeon management to assist Rud's efforts. During the raid on the Keild labyrinth, he further reveals his true nature and capabilities, reinforcing his role beyond combat. His background prior to becoming a guardian remains unspecified.

Marius's journey shifts him from an isolated guardian defined by combat to a cooperative figure integrated into Rud’s endeavors, highlighting his unique sentience among labyrinth entities.