A nemesis of Superman created by Jerry Siegel and published by DC Comics. Mr. Mxyzptlk first appeared in Superman #30 in 1944.

Amidst all of the world saving and high adventure that is the trademark of superhero comics, writers occasionally take a moment to insert a bit of lighthearted fun. To this end, a number of comic book characters have sprung up over the years to confound our most beloved heroes with their antics. The Fantastic Four and other members of the Marvel Universe have had to deal with the Impossible Man. Batman for years was plagued by the efforts of his "number one fan" Bat-Mite. During the 1980's, Superman battled the teleporting menace of the Ambush Bug. But one of the earliest examples of this type of character was another Superman villain, the imp from the 5th Dimension, Mr. Mxyzptlk.

In his first appearance, Mxyzptlk was actually named Mr. Mxyztplk. He appeared as short man with white hair, dressed in a sleeveless orange jumpsuit with purple trimming and sporting an orange bowler. He caused widespread havoc about Metropolis and when confronted by Superman, revealed that he was from the 5th Dimension (though he was unfamiliar with Marilyn McCoo) where he was a court jester. He also revealed that he could only be made to return to his home by saying "Klptzyxm" and in saying that, he disappeared. Mr. Mxyzptlk was said to be able to return every ninety days during which he would cause havoc in the life of Superman, but his method of return was always the same: saying his name backwards. After his initial appearance however, the name of the character changed to his present name switching the "p" and the "t" in his name.

After the Crisis on Infinite Earths and the reboot of the DC Universe, Mr. Mxyzptlk was reintroduced by John Byrne into the Superman mythos. Appearing very similar to his Silver Age look, Mxyzptlk began to wreak havoc in Metropolis until he agreed that with Superman that he would return if Superman could get him to say his name backward. Superman tricked him into doing this, but unlike before in subsequent appearances, Mxyzptlk was able to set the conditions of his return which have varied from having Superman get the imp to paint his face blue to having Superman hit Lex Luthor. Mr. Mxyzptlk was responsible as well for the reintroduction of red kryptonite to the rebooted universe.

Mr. Mxyzptlk used his powers to give a portion of his abilities to Batman's archvillain, the Joker allowing the Joker to rework reality with him as emperor for a time. Mr. Mxyzptlk and Bat-mite ended up in 1999's World's Finest #6 getting into a heated argument over who was better, Superman or Batman, and laying waste to this and many other dimensions. In a crossover with Warner Brothers, Mr. Mxyzptlk and the WB character Do-Do switch dimensions bringing about a meeting of two cultural icons in the four issue series Superman & Bugs Bunny in 2000.

When a new animated series was produced by Warner Brothers starring the Man of Steel, Mr. Mxyzptlk became a character as well. Voiced by Gilbert Godfried, Mr. Mxyzptlk was portrayed very similar to his comic book character. He was also portrayed in by a real life actor in Lois and Clark: The New Adventures of Superman. Mr. Mxyzptlk was played by Howie Mandel.

Source of Some Facts from http://www.supermanhomepage.com/comics/comics.php?topic=special-reports/mxyzptlk

My thanks to a noder who will remain nameless for the info on Howie Mandel's link to Mxyzptlk. Their identity will be withheld rather than brand them with knowing such info.

Howie Mandel... shudder