If you haven't seen the first episode, then there is a slight spoiler within my theory. Nothing major, though.

Probably incorrect, but I have a fun theory about her. She's much more solid and real looking than the other ghosts. What if she's in a coma? Having an out of body experience while trapped in a coma.  So far, the only flashback we have seen is of her lying down with a head injury. No confirmation of death.