Mr. Mxyzptlk is a chronic Superman harasser, who hails from the fifth dimension. He's an odd-looking, diminuative fellow who sports two tufts of white hair on either side of his otherwise bald head, a miniscule chapeau resting atop his pate, and that's just the beginning of the weirdness.
Reportedly inspired by Al Capps' Joe Blyfstyk and Bugs Bunny, the character's chief power is scientific knowledge so advanced it's completely inaccessible to residents of Earth. Many of his tricks have been described as magical. The creation of Red Kryptonite, forcing Superman and The Flash to compete in an around-the-world foot race and mustering up a creature named Bada-Bing-Bada-Boomsday, which imitated another of Superman's nemeses, Doomsday, are among Mr. Mxyzptlk's bizarro feats.
Having to be taught the values and practices of earth, Mr. Mxyzptlk turned to Lex Luthor to learn the concept of how to effectively execute a lie.
In order to reverse the effects of his trickery, Superman has to force Mr. Mxyzptlk to utter his own name backwards, "Kltpzyxm!" When he executes this shades-of-Rumplestiltskin task, his magical powers are undone.