Millie is the plain but good-natured wife of Chrestomanci.She did not originate in this world, but instead she is a former Living Aspect of the Goddess Asheth in another world that Christopher Chant visited as a child. He promised to bring her something in exchange for an Asheth temple cat, and settled on a handful of books from a schoolgirl series about a girl named Millie. The Goddess found these books so exotic and exciting that she decided no matter what, she had to find a way to go to a boarding school herself.
When she escaped from her world, the head temple attendant, Mother Proudfoot, found her in Series 12 helping Christopher defend the castle. There she gave Chrestomanci Gabriel DeWitt a trunk full of uncut diamonds to provide for Millie's education in that world.
Millie also appears in Conrad's Fate, where as a teenager school runaway, she had become trapped in a Probability Fault by a renegade & unpleasant witch. Christopher and Conrad rescued her and they went back to the castle together to study and grow up. After marrying Christopher, she and he had two children, Roger and Julia.