Poltergeists are natural phenonema related to, but not limited to, the unexplained act of items moving around on their own, sentient noises and physical attacks on witnesses. Since they have never actually been seen, they have not been proven to exist. They are often lumped in with ghosts as both are invisible and can cause mischief.
Filch is a poltergeist in Sonic the Comic. The member of the Sky Pirates was shot by Captain Plunder when the lizard took his last biscuit. The murder has given Filch a new lease of life, so to speak, granting him an increased role in Sky Pirate heists. As a poltergeist, Filch can move things without touching them (since he can't actually touch anything). This came in handy on a raid of Doctor Robotnik's ship, the Robotnicus, with Filch phasing into the secure safe and "pulling" a lever that opened the door from the inside. Despite this, Filch has never used these powers again and specifically mentioned a lack of being able to do anything useful as an excuse to get away from the Spice Maidens and learn about King Akotek.