Slimer is a ghost made up of pure ectoplasm and the first ghost captured by the Ghostbusters. Slimer is well known for his gigantic appetite and the slime he leaves behind when he passes through solid objects.
Slimer was a legendary ghost to all on the original staff of the Sedgewick Hotel. His usual territory was the twelfth floor, but his outings were usually non-violent and simply involved eating food. As a result, the hotel was able to keep a lid on the supernatural problems (for a while). For some reason, Gozer's approaching time of arrival provoked Slimer (and many other ghosts) into being much more active than usual. Eventually the staff couldn't keep the ghost a secret anymore and called the Ghostbusters. However, the Ghostbusters had not yet fully tested their equipment and weren't completely prepared for a full capture. Ray was the first to find Slimer, but missed him and Slimer escaped into the next hall, where he slimed Peter Venkman.
After many errant shots and the resulting destruction, Slimer flew into the Alhambra Ballroom where the Ghostbusters made another attempt at capture. Though Slimer is at first tossed around by some more missed shots, the Ghostbusters eventually manage to get him in the streams. Slimer was then caught in the Trap and put in the storage facility. However, he was released with the many other ghosts when the storage facility was shut down. Slimer occupied a hot dog cart on on 1221 Avenue of the Americas in front of the Rockefeller Plaza. When the Hot Dog Vendor opened the cart, he was naturally shocked to see a ghost. After the Ghostbusters defeated Gozer and left Central Park West, Slimer was sighted in the area.
Slimer seemed to be a pet to the Ghostbusters, though it's unknown what he had been doing since the Ghostbusters were put out of business years ago. Slimer did resurface after the Ghostbusters started up again and had apparently gotten much fatter in the span of five years. Throughout the Vigo incident of 1989, Slimer interacted with Louis Tully. The two first met when Louis caught Slimer eating his lunch, to which the two of them fled in fear. On New Year's Eve, Louis decides to take up a Proton Pack and help the Ghostbusters.
In an attempt to patch up their initial misunderstanding, Slimer helped an exhausted Louis by giving him a ride to the Manhattan Museum of Art (to which Louis wonders how he got his license). It seems likely Slimer had simply commandeered an abandoned bus from the havoc resulting from Vigo's Mood slime assault.
Ghostbusters The Video Game
At some point, Slimer was recaptured and kept in a Paranormal Containment Research Tank in the Firehouse in the lobby. Egon Spengler used him for various paranormal studies (sometimes noted that this is a nod to The Real Ghostbusters shows take on Slimer). On Thanksgiving weekend 1991, Slimer was playing with Peter Venkman's ESP cards inside Paranormal Containment Research Tank.
The Psi Energy Pulse damaged the Tank and Slimer escaped. The team finds him looking at the Containment Unit. When the Rookie takes a shot, Slimer avoids it and the Proton Stream hits the Unit, releasing the Sloth Ghost. Rookie and Ray pursued Slimer into the Sub-Basement but it escaped through a wall. After the Sloth Ghost was recaptured, the Ghostbusters headed to the Sedgewick Hotel on Ray's hunch Slimer would return to his old haunt. Sure enough, Slimer was found by Ray, Peter, and Rookie eating and drinking off someone's tray in front of Room 1218. Rookie took a shot but Slimer ran for it and tried to hide. Rookie roused him out and Slimer flew off towards the elevators where he descended back to the ground floor (but not before sliming Peter for a second time). Slimer is joined by Bellhop Ghosts. With renewed paranormal activity, Egon opts to split up. Rookie and Peter are tasked with recapturing Slimer. Peter believes he's feasting in the Alhambra Ballroom but the manager John O'Keefe bars entrance. Peter regroups and leads Rookie through the kitchen to access the other entrance to the ballroom. Rookie and Peter successfully traps Slimer.
Slimer is placed back into a working Paranormal Containment Research Tank when the team returns from Times Square. When Peck shut down the containment grid again, Slimer is among the escapees. After the Ghostbusters destroy Ivo Shandor and return to the physical plane, Slimer slimes Ilyssa Selwyn just as she and Peter are about to kiss in Central Park.
He is more like a pet in this version evidenced by three dog food bowls with him in the Paranormal Containment Research Tank. Much like the Realistic Version, he escapes from his tank when the Psi Energy Pulse passes through the Firehouse. However, he only stays shortly to waive goodbye to the Ghostbusters then leaves for the Sedgewick Hotel. Peter and the Rookie recapture him, and he is placed into another research tank. When the ghosts are released from the Containment Unit, Slimer escapes again, but at the end of the game, Slimer doesn't slime Ilyssa. Instead, he reappears above the garage bay in the Firehouse. The credits roll while the Rookie(s) try to catch him again.
Slimer returned in the 2016 reboot Ghostbusters film. This time he is first seen he eating all of the food out of a food cart before driving off with the Ecto-1, leaving the Ghostbusters out on the street. Later, while Slimer was joyriding with his girlfriend and some other ghosts, the Ghostbusters cause him to drive into a dimensional portal while detonating a nuclear reactor device on top of the vehicle. This causes a nuclear explosion within the portal which reverses it's polarity, turning the portal into a giant ghost trap. Slimer is last seen with his girlfriend after being flung out of the portal from the explosion, and then being sucked back into it while they both scream with joy.