Diana the Inconsistent Ghost
Or demon, or projection, or whatever she was.
She had a physical form, she could mess with light bulbs, cut the electricity, pick people up and slam them down ... but she couldn't take Sophie's (the mom's) pills from the medicine cabinet and flush them down the toilet? She knew, too, that the pills were her enemy. A couple of times she menaced Sophie when Sophie was gonna take her meds. Why didn't Diana get rid of the meds?
Could Diana operate in shadow, or only pitch darkness? Sometimes we see her in a doorway that is shadowed, but when our heroes are only protected by flickering candlelight, that tiny glow keeps her away. Couldn't she come up behind you and spear you with her long fingers? And how did she get around? She manages to travel from Sophie's house to a warehouse, or to Rebecca's apartment -- does she walk from place to place? She seems to be able to move even in light, except invisiblely; for example at the warehouse when the lady was turning the lights on and off, and Diana would get closer. But she couldn't manipulate things in the light, only ... walk? That makes no sense.
And could she be anywhere that is dark? Like, in your shoe? In your car, crouching in the space between the dash and the passenger floorboard? Inside a closed drawer? She could be under your bed or in your closet ... but not in the medicine cabinet? It seemed to be a place where her physical form would fit, but doesn't she reach out from between the slats of a wall at one point?
Was she intelligent? Sometimes she seemed to have the intellect of a child -- scratching stick figures into the floor, or her childish determination to keep Sophie to herself. Other times she seems to have an adult intellect, to the point of setting complex traps (tricking Rebecca and Martin into going to the basement and then locking them in), or stalking the dad/husband to his work.
I wish the movie had spent more time explaining the 'rules' for Diana, and less time showing us how Rebecca didn't want to be involved with a boyfriend or family.