Muller is an elven adventurer renowned as the "Golden Swordswoman," identified by her vibrant blonde hair and blue eyes. Her history prior to meeting the main protagonists is largely undefined, though she operated as a skilled adventurer in Azpire and trained as an apprentice under the mage Lemia until completing her studies.

Originally a solitary individual who preferred operating alone, her disposition transformed after encountering Taichi Nishimura and Rin Azuma. She developed strong camaraderie with them, forming a particularly fast bond with Rin due to admiration for Rin's rapid mastery of magic. This marked a significant shift in her social behavior, leading her to spend considerable time with them and value collaborative efforts.

Her combat expertise centers on dual elemental magic, specializing in fire and earth, which she integrates with swordsmanship. She executes techniques such as "Spellsword: Scorching Heat Slash" and "Red Lotus." Her high proficiency enables her to undertake A-rank adventurer missions. Standard equipment includes a long sword and dagger.

Emotionally, she harbors feelings for Taichi, culminating in a documented kiss within light novel narratives. Concurrently, nuanced interactions suggest potential romantic interest in Rin, though the full nature of this dynamic remains ambiguous. Her character arc illustrates a transition from self-reliance to valuing interdependence, influenced by her deepening relationships.