The Tightrope Girl, whose mortal name may have been Sally Slater, Lillian Gracey, or Daisy de la Cruz, was a human girl who is now a ghost.
The Tightrope Girl was once a human girl who died while performing a tightrope-walking act with a frayed rope, over a body of water of some sort; halfway across, the rope snapped, and Gracey plunged straight into the mouth of an alligator waiting below. Like most ghosts in the Haunted Mansion, beyond these basics, stories and rumors about her mortal life are many conflicting, from a witch and seductress to the innocent victim of a jealous witch curse to a ditzy but harmless relative of the owners of the house.
One thing is sure, however, and that is that the ghostly tightrope walker became a permanent resident of the Haunted Mansion, her fate memorialized in one of the Stretching Portraits. At any rate, in death, the Tightrope Girl seems to be an important ghost, and is "Chief Spookswoman" and "Membership Committee Chairghost" of the Committee of Wandering Ghosts. She is well-liked around the Mansion, including by the Organist, Victor Geist, who seems to have a crush on her.