Slimer (Earth-8591)

Slimer, also known as Onionhead, Little Spud, the Green Ghost, and Disgusting Green Blob, is a Class 4 ghost made of pure ectoplasm, and the first ghost to be captured by the Ghostbusters. He is infamously known for his mischief, shenanigans, and neverending appretite, leaving a trail of ectoplasm whenever he phases through doors and walls.

Biography
Slimer was a legendary ghost whose story was spread as an urban legend through New York City, as he haunted the Sedgewick Hotel. He made the twelfth floor his home, but his hauntings were never violent, as he only wanted food. As a result, the hotel was free from supernatural problems. However, the reemergence of Gozer led to ghosts, including Slimer, to act more constantly and more aggressively than ever more, and so Sedgewick's staff was unable to continue keeping the existence of Slimer a secret.