As a ghost, she is well known for selling demon powder, faerie blood, and other strange and supernatural merchandise. Old Molly, like many ghosts, did not accept money as her payment for magical items; instead, she asks for old, gold rings. All ghosts have a talisman—sentimental item, a lost piece of their past—that keep them tied to the living world and would allow them to pass on if found; in Old Mol's case, it was her wedding ring. It was common belief, according to Magnus Bane, that the ring was long gone, buried under the silty bed of the Thames, but Molly still accepted the bags of rings given to her in the hope that one would turn out to be hers.
In 1878, Magnus sent Will on multiple trips to her on errands to fetch ingredients for his spells. When Will was looking for a stock on yin fen, Will went to her hoping she would have some. However, something had frightened her so badly—most likely Axel Mortmain who was going to extremes to deprive the London Institute of the drug—that she wouldn't even come out of her grave.
She is an old woman who wears a ragged old dress with a white apron. She has hollow eyes, yellow-gray hair, and rough, red hands. Old Molly is apparently nearly opaque, while most ghosts are translucent—which she admitted takes some effort on her part to appear that way.