Real name Marjorie Potts.
Tottering on high heels that look sadly incongruous on a woman in her fifties, she is always handy as a source of well-handled used tenners and much-needed square meals for the Witchfinder Army, which in its turn refers to her as "the Painted Jezabel" and "the Whore of Babylon".
On all days except Thursdays, she exerts such discipline as she still can for a handful of old clients who turn up mainly out of habit or because they need a chat.
On Thursdays, Madame Tracy Draws Back the Veil with the asistance of her spirit guide Geronimo, who she fondly believes is the discarnate spirit of a wise and compassionate spiritually evolved North American Indian. Newton Pulsifer knows she is correct about the North American Indian part, yes, but privately considers she could have done some less sketchy research.
Shadwell tends to hammer on the walls during her seances, as he considers spiritualism is one dangerous step too close to actual witchcraft (why else does the Holy Bible call a spirit medium the Witch of Endor, hey? (Samuel, chapter 28:4–25) And Holy Scripture is never wrong, especially when it commands us not to suffer a witch to live! (Exodus 22:18)) and it affronts his Witchfinder's soul. Tracy doesn't mind this, as it gives her seances an air of added realism.
Ths clash of lifestyles and the applied theology resulting from it can be found in Good Omens.