From what I can recall Sad Lisa was inspired by a young woman he met while confined to a tuberculosis facility NOT a mental institution. While the song's a little cryptic, I don't think it's in reference to a ghost or an apparition. Seems more like he's observing a young woman's struggle w/emotional conflict(s) of some sort. Where he says 'No one can see her' it would seem he's speaking of her 'true' nature. Wandering the halls doesn't suggest a ghost to me, but a metaphor for being a little lost or directionless. As most of us have experienced when living through a trama. Loss of a spouse, childhood abuse, etc. As the narrator he hopes to free her of her pain...whatever it is. That's what I get out of the song, anyway.