Tante Charlotte is Mary's great-aunt and adoptive guardian, providing her a home on a northern English country estate. She initially appears as a conventional elderly caretaker managing the household with staff including the gardener Zebedee and housekeeper Miss Banks.

Her past reveals she was once a young witch called the "Redhead Witch" who attended Endor College. There, she distinguished herself academically and possessed the distinctive red hair later seen in Mary. During her studies, she discovered the rare fly-by-night flower on campus, attracting the attention of Headmistress Madam Mumblechook and Professor Doctor Dee. Witnessing their catastrophic failed experiments using the flower's magic to transform humans into witches, she stole the remaining fly-by-night seeds and fled the college. This severed her ties to the magical world, leading her to abandon witchcraft for an ordinary life.

When Mary becomes entangled with Endor College after discovering the same flower, Charlotte's hidden history resurfaces. She contacts Mary through a magical mirror within an enchanted cottage—her former home—revealing her past and warning of the flower's dangers. She urges Mary to use the last fly-by-night bulbs to return home safely, prioritizing Mary's well-being over confronting the college directly.

Charlotte's legacy directly shapes Mary's choices; her experiences underscore the film's themes of self-reliance and rejecting shortcuts. Though unable to intervene physically, her guidance empowers Mary to confront Endor's threats and ultimately reject magical solutions, mirroring Charlotte's own life path.