Stay Puft Marshmallow Man

The Staypuft Marshmallow Man was originally the vessel used by Gozer to destroy the world in Ghostbusters but became a ghost in its own right shortly afterwards (despite having been blasted in the film).

The Staypuft Marshmallow Man has varied in the series from a villain to a hero but rarely displayed the same level of malevolence as it did in the original movie - perhaps due to the fact that it was no longer fully controlled by Gozer.

Role In Ghostbusters (original film)
The Staypuft Marshmallow Man was formed during the climax of the original Ghostbusters movie in which the destroyer-god known as Gozer decided to take the form of the first thing the Ghostbusters thought of to destroy their world - despite the group making an effort to clear their thoughts Ray couldn't help himself from thinking of the one thing he felt could never possibly harm them: namely the Staypuft Marshmallow Man.

Gozer caught Ray's stray thoughts and formed a gigantic Staypuft Marshmallow Man by which to terrorise New York - this giant Staypuft Marshmallow Man would proceed to climb the skyscraper the Ghostbusters were on in an attempt to reach them: proving itself durable despite taking heavy damage from the group's photon-packs.

In the end Egon decides the only way to defeat the Staypuft Marshmallow Man is to "cross the streams" - which risked a cataclysmic explosion but surprisingly Egon's gamble paid off and the resulting explosion was strong enough to destroy the Staypuft Marshmallow Man while not causing a cataclysm (though it did result in a river of molten marshmallow raining down on the streets below)