A ghost is an undead being, Summon-able by Robotguy. Although, they do not immediately do his will, regrettably. This is due to the free will they acquire due to leaving their fleshy prison. When you die, you will become a ghost, due to your soul being placed inside your, metal body. Occasionally, a zombie will give up a ghost. Ghosts are very resistant to everything, but they cannot move anything either. A good tactic is to just scare the opponent.
Light, makes them disappear,
Water, due to it being constantly moving,
Salt, burns them,
Cross, Bible, Holy water.