First off, amazing puzzles! As i myself always sucked at making puzzles(but love solving them) it always amazes me when someone else is able to come up with such things.
The idea itself is also neat, however i really disliked that some walls were invisible in physical-mode, it made no sense thematically and was just annoying for no real benefits for a puzzle challenge or anything. But that is really my only problem with the game.
Well, there is also the key inputs. As i said in another game today, it's not your fault, but many european keyboards have the Y and Z key switched, meaning for us having the keys at that location is really uncomfortable and even painful for a game like this.
And one really really really minor nitpick, in the introduction level, you could have a box lying around so that the player can test the possession feature. Just so, you know, the player knows what the instructions are talking about. But like i said, that's just nitpick.
I had really fun with this.