I have mentioned this on other posts, but yes, I have seen a ghost many times. Grew up in a house with one.
Never harmed us, but needless to say, just the presence scared us. I was a teenager, and not like I could move at the time.
The rest of my family saw it, and heard it, including our pets.
This one thing I can with a certainty. There are "ghosts" or whatever you want to call them. This was not of this world, for sure.
It would make footstep noises when you couldn't see it, though it seemed to glide by if you did see it. It was upstairs, as well as downstairs. It went through walls, and certainly was watching us from time to time.
Do I think it was a demon? No. Do demons exist? I think so, but I have not seen one, unless I count my ex-wife.