Slut

Slut is one of the many mutants living beneath New New York, glimpsed among the crowd whenever the show ventures into the sewers Leela's parents call home. She's rarely more than a face and a name, but the name itself is the joke — delivered as flatly as any other introduction, as if nobody down there finds it odd at all.

The gag is entirely structural: a throwaway character whose only distinguishing feature is a title so blunt it becomes funnier the more casually it's treated. Tress MacNeille tosses off her lines with total nonchalance, and Futurama's commitment to never acknowledging the absurdity is what makes Slut a small, perfect piece of background comedy.