An imp from the Fifth Dimension of Zrfff with magical abilities that easily affect Superman. He can be forced back to his home dimension (most chronicles say for a period of one month) if he says his name backwards ("Kltpzyxm"). His many pranks have impacted the Superman family for decades and yet all his topsy-turvy magical, nightmarish alterations of reality return completely to normal once he has been banished.
In October 1944 he appears on Earth doing countless pranks and meets Superman when he was bandaging the mayor at a council meeting after making him appear ill by making him speak "like a jackass". He later explains how he is from another dimension where his full-time activity is a court-jester. After "poking his nose into the secret volumes of a brilliant scholar" (he's depicted holding a book named "Mxyztplkology") he learns "two magic words, one of which would transport him to this dimension and the other word if spoken aloud would return him to his world for a time" (S No. 30/3, Sep-Oct 1944: "The Mysterious Mr. Mxyztplk!").
The imp comes back to plague Superman repeatedly, a few examples include adventures where he: becomes mayor of Metropolis (S No. 96, Mar 1955: "Mr. Mxyztplk -- Mayor of Metropolis!"), concocts a spell to relegate Superman to his weird dimension (Act No. 208, Sep 1955: "The Magic of Mr. Mxyzptlk!"), has an alarm in his hat to warn him when he begins to say his name backwards (S No. 131, Aug 1959: "The Menace of Mxyzptlk!"), comes up with an idea to remain underwater to prevent himself from speaking (S No. 154, Jul 1962: "The Underwater Pranks of Mr. Mxyzptlk!"), and resolves to curse Superman with powers similar to his own (S No. 171, Aug 1964: "The Curse of Magic!"). Later, Mxyzptlk is also one of Superman's most powerful foes summoned by Mr. Xavier to distract the Man of Steel (S No. 299, May 1976: "The Double-or-Nothing Life of Superman!").
Superboy meets a younger Mxyzptlk (not yet mostly bald and with red hair) earlier in his career (for example, SB No. 83, Sep 1960: "The Dreams of Doom!", SB No. 120, Apr 1965: "The Invulnerable Imp!"; SB No. 131, Jul 1966: "Lex Luthor, Imp!").
In July 1980 Superman returns from an interstellar mission to discover that the world he knows has been replaced by one in which everyone has been reversed with a counterpart of the opposite gender. Instead of a female Lois Lane this world's Daily Planet has a male reporter named Louis Lane. Instead of Jimmy Olsen there is Jenny Olsen. Wonder Woman is replaced by Wonder Warrior, and so on. And the greatest hero on this world is Superwoman. There is also a Clara Kent who strangely is not the same person as Superwoman. In fact unlike her male counterpart in the real world, Clark Kent, Clara Kent is just an ordinary person. Based on this fact, as well as the fact that a file on Superwoman's enemies shows a male Mxyzptlk, Superman is able to deduce that Mxyzptlk is behind this madness. Mxyzptlk is too vain to alter himself in any way so even in this world of reversed genders he is still male. Superwoman and Clara Kent are different people because Mxyzptlk, despite his incredible powers, has never discovered that Superman and Clark Kent are one and the same. Superman confronts Mxyzptlk, and uses the magic lasso taken from Wonder Warrior to force Mxyzptlk to say his name backwards causing him to return to the 5th dimension and undo his spell over the earth. When Superman returns to the Daily Planet, he is shocked to meet a Louis Lane, but its not the same person from the gender reversed Earth -- it's Lois's cousin from Pittsdale (S No. 349: "The Turnabout Trap!").
(See also Mr. Mxyztplk)