Marller, also known as Mara or Marla, is a first-class demon and the first demon to appear in the story. She serves as a recurring antagonist to Keiichi Morisato and the three main goddesses, Belldandy, Urd, and Skuld. Marller is a proud and competent employee of the Office of Demonic Affairs, and her official duty is to increase the demons' market share on Earth by signing contracts with humans. However, she often neglects this primary objective in favor of her self-appointed side duties: disrupting the goddesses' activities, making them suffer, and ultimately driving them away from the mortal realm. She operates under the assumption that if the goddesses are removed, the demons' market share will automatically reach one hundred percent.

Marller's personality is a blend of mischief, determination, and insecurity. She initially appears devious and underhanded, but she does not genuinely take pleasure in causing suffering. Instead, she is more of a mischievous and persistent thorn in the goddesses' side. Her relentless focus on thwarting the goddesses borders on tunnel vision, which stems from a painful past: she was once a childhood friend of Urd, and the two were very close. When Urd chose to fully embrace her identity as a goddess, the friendship ended, leaving Marller feeling abandoned. This sense of betrayal fuels her rivalry, though she still seems to enjoy the company of the goddesses when circumstances allow for casual interaction. Despite her demonic status, Marller is not truly cruel; she is more determined and mischievous than malicious.

In the television series, her character gains additional depth through moments of vulnerability. She suffers from insecurity and self-doubt, particularly regarding her inability to increase the demons' market share due to the goddesses' constant interference. Urd's blunt criticism that she has no talent brings Marller to tears, revealing a fragile ego beneath her bravado. This insecurity sometimes leads her to act impulsively or incompetently, resulting in a string of failures that become a running gag throughout the series. Nevertheless, she remains loyal to her superior, the demon lord Hild, and continues to carry out her orders with unwavering dedication.

Marller's key relationships are primarily antagonistic, with her most significant bond being the one she once shared with Urd. Their history adds a layer of personal stakes to their conflicts, as Marller's actions are partly motivated by a desire to prove herself or to retaliate for Urd's choice to become a goddess. She also interacts frequently with Belldandy and Skuld, often targeting them with schemes that backfire in comedic ways. On the demonic side, she maintains a subordinate relationship with Hild, who trusts her enough to assign her important tasks despite Marller's frequent failures.

Notable abilities include a high degree of competence in demonic magic. Marller can transform humans into objects, as demonstrated when she turned Keiichi's sister Megumi into a car. She also created a trio of diminutive female ninja operatives, apparently transformed from rats, to serve as her agents of chaos and skilled fighters. Additionally, she has two notable weaknesses: she cannot stand good luck charms, and she is compelled to dance uncontrollably whenever she hears rock or disco music. These quirks often undermine her schemes when the goddesses or their allies exploit them.

Throughout the story, Marller undergoes minimal outright character development, but her portrayal in the television anime fleshes out her insecurity and self-doubt, making her more sympathetic than a typical villain. She remains a consistent source of conflict and comedy, always trying to undermine the goddesses' happiness while secretly longing for the friendship she lost. Her role is that of a persistent, often bumbling antagonist whose loyalty to Hell and personal grudge against Urd keep her a recurring presence in the lives of the main characters.