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.
- Charlie from It's Always Sunny in Philadelphia. Dennis is occasionally this, but most if the time he is completely Axe-Crazy.
- Lenny and Squiggy from Laverne & Shirley.
- Skippy or Nick from Family Ties.
- Reverend Jim and Latka from Taxi.
- Klinger from M*A*S*H.
- Spencer Shay from iCarly.
- Sinjin van Cleef fromVictorious.
- Kramer from Seinfeld.
- Cody, then Jean-Luc, from Step by Step.
- Harry from 3rd Rock from the Sun.
- Jack from Will & Grace.
- Steve Urkel and Waldo from Family Matters.
- Sheldon from The Big Bang Theory.
- Kimmy Gibbler from Full House.
- Barney Stinson from How I Met Your Mother, with a heaping help of Jerkass, and a handful of Deadpan Snarker.
- Beans from Even Stevens.
- Alistair from As Time Goes By.
- Boner from Growing Pains.
- Jazz from The Fresh Prince of Bel-Air.
- Eric from 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.
- First Amp the nerd and then Rembrandt the surfer on "Superhuman Samurai Cyber Squad".
- Oddly enough, Star Trek: The Next Generation has one: Q.
- Maynard G. Krebs from The Many Loves of Dobie Gillis.
- 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, the Cosgrove Hall series.
- Jay's father Franklin Sherman from The Critic.
- Stanley on The Amazing Chan and the Chan Clan.
- Milo on Fish Hooks.