Grande-Tante Charlotte serves as the elderly guardian and great-aunt to protagonist Mary Smith, presiding over the northern English country estate where Mary stays. She manages the household with unobtrusive yet firm command, aided by staff like a housekeeper and gardener.

Charlotte's past reveals she attended Endor College, a magical institution, where her distinctive red hair marked her as a powerful witch. While there, she discovered the rare "fly-by-night" flower on Endor's grounds. This act ignited the obsessive pursuit of Headmistress Madam Mumblechook and scientist Doctor Dee, who sought the flower's power to transform all humans into witches.

Witnessing the disastrous failure of their experiments—including the monstrous transformation of a fellow student—Charlotte stole the fly-by-night seeds and fled. In the frantic escape, she lost both the seeds and her broomstick, severing all ties to the magical world. She subsequently abandoned witchcraft entirely for an ordinary life.

Years later, when Mary is drawn back into Endor's orbit through the same flower, Charlotte uses an enchanted mirror in her isolated island cottage to send Mary visions. Through these visions, she discloses her past, warns Mary of the flower's danger, and urges her to use the remaining seeds to return home safely. Despite this caution, Charlotte's account of her experiences ultimately empowers Mary to challenge Endor's corruption and rescue her friend Peter.

Charlotte's story concludes as a supportive guardian, her history functioning as both a warning and an unintentional catalyst for Mary's self-reliance. Her abandonment of magic highlights ordinary courage.