The outrageous character who undermines all of a sitcom's attempts at verisimilitude (such as they are).
Often, the outrageous character(s) clearly has a key to the residence of the more central characters of the show, since they nearly always show up unannounced and never knock or ring the door bell (see Drop-In Character, Your Door Was Open and Inadvertent Entrance Cue).
The Wacky Guy may also be a Weirdness Magnet, his own strangeness an attractor for other weirdness without being directly responsible for it.
- Lenny and Squiggy, Laverne and Shirley.
- Skippy, Family Ties.
- Reverend Jim, Taxi.
- Spencer Shay, iCarly.
- Sinjin van Cleef, Victorious.
- Kramer, Seinfeld.
- Cody, Jean-Luc or Rich Step by Step.
- Harry, 3rd Rock from the Sun.
- Jack, Will and Grace.
- Steve Urkel, Family Matters.
- Kimmy Gibbler, Full House.
- Barney Stinson from How I Met Your Mother, with a heaping help of Jerkass, and a handful of Deadpan Snarker.
- Beans, Even Stevens.
- Alistair from As Time Goes By.
- Boner from Growing Pains.
- Jazz from The Fresh Prince of Bel-Air.
- Eric on Boy Meets World became this after he Took a Level In Dumbass.
- On Smart Guy, Moe, Mackie and Yvette's friend Nina have this trope covered between them.
- Nora Valkyrie from RWBY.
- Master Shake from Aqua Teen Hunger Force seems to be a combination of this trope and a Jerkass.
- Wakko from Animaniacs
- Dale Gribble and Bill Dauterive from King of the Hill
- Mr. Toad from The Wind in the Willows, especially so in the Cosgrove Hall series.