Miriam is the mother of Jessica Reeves.
Miriam died in a car accident as a teenager, while pregnant with Jessica (who was called "Jesse" for short). Jessica's mother was raped by an unknown man before she died, leading her to become pregnant.
Because of her bloodline's witch powers, Jesse repeatedly saw the ghost of Miriam during her lifetime. Miriam usually appeared in troubled periods of Jesse's life, comforting Jesse in her times of need.