Ghost is one of Kirby's many copy abilities, a rather special one. It has so far only appeared in Kirby: Squeak Squad.
Ghost can only be copied from the unique mini-boss Tedhaun. Tedhauns replace one mini-boss from each world after all Ghost Medal pieces have been unlocked. They are found in large treasure chests. Ghost bubble can be made by mixing two Sleep bubbles.
Ghost is a somewhat useless ability. You can't go down pass-through platforms, possessed enemies can't be taken outside the room they were found in, possessed enemies weigh Kirby down, plus you can't possess Squeakers. It also turns Kirby into a punching for bosses and min-bosses.
Attacks and Features
With Ghost, Kirby can dive into regular enemies and control them from inside. Ghost Kirby also floats in air without losing altitude, similiar to UFO, and like it, Ghost Kirby cannot use stairs or move through pass-through platforms.
Attack button = Make a quick and very short dash directly forward. If Kirby hits a regular enemy, he can control it until the enemy is destroyed, but Kirby doesn't receive damage. Doesn't work on bosses or mini-bosses.
Other attacks = Kirby can use the attacks of the enemy he's controlling. If he controls an enemy with no special powers he can't use any attacks at all.
A thing that often happens with ghost is that you cannot go through platforms when in this mode, so you end up having to get rid of the rare ability to continue.