Marielle is a French witch of unknown coven affiliation, presenting as a woman in her twenties despite centuries of life. She possesses average height, a voluptuous figure, fair skin, wide dark brown eyes, full lips typically adorned with pink lipstick, and black hair in a bob-cut, frequently wearing pink nail polish. Her attire varies: a light green hooded robe at the Black Market, a light pink jumper with a red coat and purple scarf outdoors, and a long black dress with a veil for coven gatherings.

Her history traces back to serving in temples of old gods, using her body as a conduit to channel divine power into men and bear children for her community. She claims existence since Marie Antoinette's execution. Previously, she attempted to read the Testament of Carnamagos to break her mentor Phyllis's curse, but the grimoire overwhelmed her mind, leaving her barely coherent. Phyllis, the coven leader, taught Marielle witchcraft from childhood, serving as both mentor and motherly figure. Marielle maintains fierce loyalty to Phyllis, defending her own rule-breaking by citing Phyllis's well-being.

Marielle first meets Chise Hatori at a Black Market auction where Chise seeks a young dragon. Marielle offers financial help in exchange for dragon blood, forming a contract. When Chise later cannot fulfill this due to the dragon's departure, Marielle examines Chise's dragon-cursed hand. She attempts a curative spell that fails and proposes Chise join her coven for specialized curse removal. After Chise declines, Marielle defies coven rules by revealing a curse-transfer method to Elias Ainsworth. Phyllis reprimands her, but Marielle argues her actions were for Phyllis's sake, threatening to leave if punished. As amends, she aids Chise by shapeshifting into a bull to pursue Joseph.

Later, Marielle discovers an injured associate of Seth Noel and assists him to the College. There, Elias enlists her to investigate a colleague drained of magical energy. She identifies the energy signature as matching the Testament of Carnamagos—the same grimoire she previously encountered and that Elias read to break Chise's curse. Marielle agrees to investigate further in exchange for a future favor from Elias.

Her magical abilities stem from her lineage as a descendant of temple prostitutes, granting unique traits: energy transference to spirits or fair folk, longevity via magic, and a maternal resonance evoking motherly associations in others. She specializes in curses involving communication spells, such as speaking with birds or unconscious individuals. She also possesses limited shapeshifting abilities, though the process exhausts her due to lack of proficiency.

Personality-wise, Marielle appears maternal yet displays immature tendencies. She leverages bargains extensively, demanding payment or favors before assisting others, even in morally ambiguous contexts. Despite this transactional approach, she demonstrates compassion for those she sympathizes with, like Chise and Phyllis, often justifying rule violations as necessary for their benefit.