It does not have a shape or form. This ghost only randomly opens/closes Toilet doors and turns off the lights. It appears in the bathroom from the 2nd Section of Main building.
In the Girl's room on the 1st floor, it closes or opens random toilet doors.
In the Boy's room on the 3rd floor, it switches off the electric light when you try to turn it on.
Some insist it's not a ghost because there is no physical entity to be seen. But it's likely that it's just a ghost that exists without a form. An example of this would be Na-hyae (Original). This could also likely be an aesthetic effect for the game.