I do not believe in ghosts as being people who didn't "pass over" or people whose life on earth was not completed. I believe that when everyone dies they go to be judged by Jesus Christ and then those who died in a state of mortal sin are immediately put in Hell. Those who die in a state of grace but in need of further purification go to Purgatory. As for those who die in a state of grace but with no further need of purification, they go straight to Heaven. In other words, once a person dies, they are in either Heaven, Hell, or Purgatory... period. So ghosts are not human souls.
I do believe it is possible for demons to appear as ghosts though and since Satan is the father of lies I think it is also possible for them to convince people that they are deceased people when they are demons.