Marller, also called Mara, is a first-class demon who once shared a close bond with the goddess Urd until their friendship soured after Urd chose to embrace her divine role. Under divine mandate, Urd and Belldandy imprisoned Marller within a catalyst—a CD—to suppress her chaotic influence. A random incident eventually shattered the seal, freeing her.

She directs her schemes primarily toward Keiichi Morisato, whom she deems a nexus of concentrated good fortune—an antithesis to her demonic essence. Convinced his elimination would restore equilibrium, she relentlessly pursues him. Though formidable, her power is counterbalanced by quintessential demonic frailties: heavenly hymns inflict pain, hard rock compels her into helpless, involuntary dancing, and she recoils from talismans or symbols of luck.

Her earthly presence hinges on a catalyst, a physical object that anchors her to the mortal realm while simultaneously acting as a prison. This tether reflects the universal mechanics binding demonic entities, necessitating contractual agreements with humans, much like those forged by goddesses. Her history intertwines with celestial-infernal tensions, particularly her fractured alliance with Urd and adversarial clashes with Belldandy.

Across narratives, Marller repeatedly incites turmoil targeting Keiichi, deploying her powers to destabilize his life. Yet her stratagems inevitably unravel, foiled by her vulnerabilities or the timely interference of divine allies. Her portrayal underscores her role as a persistent antagonist, fueled by both personal grudges and the intrinsic malevolence of her demonic heritage.