Slimer is a recurring character within the Ghostbusters franchise. Ranked as a Class-5 Full-Roaming Vapour by Ghostbusters Inc., Slimer is a fat, disgusting blob of a ghost made up of pure ectoplasm. He was the first ghost that the Ghostbusters ever captured.
Slimer originally haunted the Sedgewick Hotel of uptown New York City. For the most part, he remained on the twelfth floor and his activities usually revolved around eating food, but in 1984, the approach of Gozer the Gozerian caused a significant increase in spectral activity across the city. Slimer had become bolder and much more gluttonous and the hotel staff were starting to notice his actions. The newly-established Ghostbusters Inc. were called in to settle the situation, though the team had yet to test their equipment and were not fully prepared for the encounter.
During the bust, the Ghostbusters caused more damage to the hotel than Slimer had. After pursuing him all over the building, the Ghostbusters finally cornered Slimer inside a dining hall on the ground floor where they were able to trap him. Slimer was later deposited in the Ghostbusters' Ecto Containment Unit. Several days afterwards, Walter Peck of the Environmental Protection Agency, convinced that the Ghostbusters' equipment was an environmental health hazard, ordered the Ecto Containment Unit to be shut down, unaware of the dangers of doing so. With the Containment Unit shut down, an explosion of psychokinetic energy occurred as all of the ghosts within the unit were freed, including Slimer.
Slimer's activities in the years following the Gozer incident are unclear and it is not known if the Ghostbusters ever re-captured him. However, he was seen five years later inside the Ghostbusters firehouse where he was encountered by Louis Tully. Both Louis and Slimer ran away at the sight of each other. Following that meeting, Louis encountered Slimer again on New Year's Eve 1989. Having donned a Ghostbusters uniform and proton pack, Louis wanted to help the Ghostbusters put a stop to the undead sorcerer Vigo but had to rely on public transport to get to the Manhattan Museum of Art. Slimer was driving the bus that Louis took to get to the museum.
In the animated series' The Real Ghostbusters and Extreme Ghostbusters, Slimer acts as the Ghostbusters' pet and company mascot. After reappearing following the destruction of Gozer, Slimer became drawn to the Ghostbusters' headquarters. Apparently he was lonely and the Ghostbusters were the only people that paid any attention to him. While the Ghostbusters initially reacted negatively to Slimer's presence, he actually proved to be helpful to them on some of their cases and the team came to accept him. Throughout the years, Slimer aided the Ghostbusters whenever a ghost was needed for the situation. Because he helped the Ghostbusters, Slimer had a large number of problems of his own. Many ghosts considered Slimer a "traitor" amongst ghosts for his compassion with the humans. This often put Slimer on bad terms with many villains who faced the Ghostbusters.
Slimer proved to be a problematic repeat haunter. He began terrorizing an apartment complex occupied by Alan Crendall, who thought it was a curse for what his uncle Janosz Poha did in 1989. One week later, Alan told Winston Zeddemore about his problem. Winston and Peter Venkman investigated the following afternoon. While Winston scanned the Crendall's unit in room 426, Peter checked next door in 427. Peter ran into Slimer and flew through the wall into 426.
An hour or two later, Winston and Peter were still having an unusually difficult time in capturing Slimer. In a shocking display, Slimer was able to pull free from the proton stream. Using Winston as bait, Peter managed to ambush Slimer and trapped him. Slimer was returned to a Paranormal Containment Research Tank in the Firehouse. Egon later postulated the current uptick in P.K.E. made Slimer stronger than usual.
In the fall, after the Ghostbusters' trip across the nation, Egon used Slimer as a test subject for his studies on the Ghost Smashers. For several days, Egon experimented on explosive dispersal of mid-range ectoplasmic manifestations. Slimer proved to be an excellent test-subject. Egon concluded the time to reconstitution in a 180 cubic foot enclosure averaged at around three hours and 47 minutes. From that data, he extrapolated when the entities atomized by the Ghost Smashers would also reconstitute. When Egon wasn't looking, Peter dispersed Slimer for fun.
During the tail end of the Tiamat incident, the battle between the Ghostbusters and the Gozer-Tiamat-possessed Ray created another chance for Slimer to escape from the Paranormal Containment Research Tank. Three months later, Slimer was seen flying past Saks Fifth Avenue. At some point after the Saks Fifth case, Slimer returned to the Sedgewick Hotel. He was subsequently recaptured and returned to the Paranormal Containment Research Tank. During the Chi-You incident, Slimer snapped at Michelangelo. Ray demonstrated the effects of certain books on ghosts. He waived Fairfax's Demonologia at Slimer who recoiled and shielded his eyes. Once the demonstration was over, Slimer made faces at Ray. Ghostbusters from a parallel dimension were displaced by a ward of protection and Proteus' spell. They went into the Firehouse unaware it wasn't their own. Ray Stantz saw Slimer in the containment tank and went to release him. Egon Spengler observed Slimer's readings on his P.K.E. Meter and noticed they were different from their Slimer. It was too late. Slimer grabbed Ray and threw him aside. Egon, Peter, and Ray opened fire and wrangled him. Winston trapped him. Slimer was returned to the confines of the tank where he began consuming a cooked turkey.
In most of his appearances, Slimer is a primal creature that acts solely on instinct, lacking any real intelligence. He has a limitless appetite and will eat virtually anything, though he tends to flee when humans take notice of him.
In the animated series, Slimer seems to have a certain degree of intelligence and understands basic human concepts. He is curious and easily excitable, and seems to genuinely care about the Ghostbusters, especially Peter Venkman (who hates him passionately). His behaviour is still largely influenced by his neverending hunger; he regularly eats the Ghostbusters out of house and home and his pursuit of snacks has led to several mishaps and near-catastrophes.
Slimer has little in the way of special abilities. He constantly levitates and can pass through solid objects, leaving behind ectoplasmic slime on everything he touches. Slimer does not appear to be capable of possessing humans like many ghosts are, or if he is, he chooses not to.