Slimer is the ghostly sidekick of the Ghostbusters, a green focused non-terminal repeating phantasm, or a class five full-roaming vapor. He is a glutton and a slob, but is very affectionate and loyal to his human friends. Originally named Onionhead due to his foul smell and vague onion shape, he haunted the twelfth floor of the Sedgewick Hotel in New York City. His precise origins are unknown. Some have theorized that he was brought forth into the human world by occult rituals conducted in the hotel basement, but this would not account for him choosing the twelfth floor to haunt; others have theorized that perhaps he is the ghost of a gluttonous guest who died while staying on that floor. Whatever his origins, Onionhead was a mainstay at the Sedgewick Hotel and a constant nuisance for both guests and staff, to the point that everyone just began ignoring him.
Ultimately, when the coming of Gozer meant that Onionhead was being even more of a pest than usual, Sedgewick manager J.M. Shupp called the Ghostbusters to take care of the problem, as quickly and quietly as possible. After "sliming" Peter Venkman, the green ghost was caught by the paranormal eliminators in the Sedgewick's ballroom after a lengthy and destructive standoff (much to Shupp's dismay). He was placed in the containment unit, where he remained until Walter Peck ordered it shut down some time later. Although Onionhead escaped with the rest of the ghosts and wreaked havoc throughout the city, he eventually returned to the firehouse, because the Ghostbusters had been the first people to actually pay him any sort of attention and he was merely harmless and lonely. Although initially the Ghostbusters reacted to his presence with hostility, the ghost ended up assisting them in dealing with evil doubles of themselves created when their old uniforms became saturated in ectoplasmic residue from the exploded Mr. Stay Puft.
The ghost was then allowed to stay on, as a sort of pet. Due to being slimed by him, Venkman was the only one of the Ghostbusters who did not want anything to do with Onionhead. Ray Stantz coined his new name, "Slimer," as a joke at Venkman's expense.