Mariel, also sometimes written as Marielle, is a witch of French origin who appears as a supporting character in The Ancient Magus' Bride. She has a voluptuous figure, fair skin, wide dark brown eyes, full lips, and short black hair cut in a bob. She works as a shepherd of goats and leads a relatively quiet life by witch standards, but her calm and worldly demeanor conceals a complicated past and a set of burdens that she eventually shares with others. Mariel is deeply connected to the wider world of witches and forbidden magic.

Her personality blends maternal warmth with an occasionally immature streak. She is willing to go to considerable lengths for those she loves or sympathizes with, even if her methods could be considered amoral. She also has a fondness for striking bargains, often requesting favors or payment before she offers help. This pragmatic approach reflects her independence and resourcefulness. Despite her sharp intelligence and strong will, she is compassionate and gentle, making her a valuable ally to the story's protagonist, Chise Hatori.

In the narrative, Mariel acts as a supporting figure who provides guidance and assistance, particularly in matters of magic and the hidden world of witches. Her relationship with Chise is built on mutual respect and a shared understanding of the dangers that come with magical knowledge. Over the course of the story, she begins to open up about her own past, revealing vulnerabilities that she had kept hidden. Her notable abilities include practical witchcraft, a talent for negotiation, and a familiarity with esoteric and forbidden magical practices.