Wacky Guy

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.

Contrast Deadpan Snarker, although they can be combined. Compare Fun Personified, Cloudcuckoolander, The Ditz. May be behind the Zany Scheme.

Fanfiction

 * In Lighter and Softer Neon Genesis Evangelion fanfics, Gendo Ikari is frequently a combination of this and Bunny Ears Lawyer.

Film

 * Batty Koda from Fern Gully

Live Action TV

 * Lenny and Squiggy, Laverne and Shirley.
 * Skippy, Family Ties.
 * Reverend Jim, Taxi.
 * Spencer Shay, I Carly.
 * 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.

Western Animation

 * Master Shake from Aqua Teen Hunger Force seems to be a combination of this trope and a Jerkass.
 * As well as Cartman and Bender from South Park and Futurama.
 * 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.