The A.V. Club has a great article on TV characters whom the writers use to comment on the oddities of the show's fictional universe. Their template for this is "Frank Grimes," a guest character on the Simpsons introduced in the episode "Homer's Enemy"
TV fans who watch a series week to week will inevitably start to think of the characters in that series in somewhat intimate terms—as people they might be friends with if those people existed in real life. Yet most TV characters are extremely heightened versions of real-life types, and usually, they’re heightened in ways that would be incredibly irritating to encounter in real life. Enter the hapless guest star, forced to contend with the series regular, whose buffoonery becomes especially outsized for that episode.
See: Frank “Grimey” Grimes, a work-a-day schlub who’s forced to share workspace with Homer Simpson in the classic Simpsons episode “Homer’s Enemy.” Homer’s lackadaisical approach to life and his extreme string of good luck drive ol’ Grimey off the deep end, with tragic results. It’s as if a realistically drawn character with constantly defeated hopes and dreams wandered into a cartoon universe and was punished for doing so, through absolutely no fault of his own. Episodes like this skew toward the dark, but “Homer’s Enemy” is one of the darkest.
I love characters like this because they force the audience to look at their favorite characters with fresh eyes. The A.V. Club article identifies 20 such characters. Do you have a favorite "Frank Grimes?"