In the script for Ghostbusters, Slimer is never called by any name, but is described as a "focused, non-terminal repeating phantasm or a class 5 full roaming vapor". The creature's original title was "The Onionhead Ghost", which the film crew dubbed him for the horrible odor he used to scare a couple in a scene cut from the original movie. Slimer’s personality is one of tremendous gluttony, and he is referred to as a “disgusting blob”. In the movies, he is not named and is portrayed as an antagonist.

In the cartoon, he is known as Slimer, is able to speak, and demonstrates a child’s intelligence and intense loyalty to Ray and the Ghostbusters with the personality of a dog.