Mister Mxyzptlk

Mister Mxyzptlk is a villain from Superman comics and media and is a classic example of a trickster villain, unlike many of Superman's enemies he doesn't seek conquest or ruin - he just wants to mess with Superman for amusement as well as teach lessons to others, albeit in the manner of a trickster-god. He served as the main villain in  What Ever Happened To The Man Of Tomorrow  and  Supergirl: Cosmic Adventures in the 8th Grade.

Origin
Mister Mxyzptlk is a troublemaking imp from the Fifth Dimension who has been harassing Superman since the early days for no reason other than boredom and slight malice - his actions often get him arrested and set to trial in his own dimension but he continues to return since punishment doesn't really affect a being like Mxyzptlk - who can just as easily escape any imprisonment or sentence he may have had imposed on himself.

Powers and Abilities
Mxyzptlk is an virtually omnipotent reality warper due to extremely advanced "Super-Science" that works like magic, allowing him to perform virtually anything imaginable - though he has not killed Superman and has a weakness in the form of whenever someone tricks him into speaking his name backwards he will be banished to the Fifth Dimension for a short while.

What Else He Is
A mischievous being from another dimension, Mr. Mxyzptlk possesses powers so advanced they can only be described as magic. He's a prankster, a troublemaker and a genuine threat. He knows Superman's double identity but would never tell, fearing it would lessen the control he'd have over the Man of Steel. Besides, threatening to reveal Superman's secret is much more fun than actually doing it. If he wanted to, Mxyzptlk could kill Superman outright by simply rearranging the Man of Steel's molecules, but again, where is the fun in that? In some weird way Mr. Mxyzptlk considers himself Superman's "pal", always showing up when least expected to throw a surprise into "the big boy scout's" life. Mxy thinks Superman actually looks forward to their running duel of wits as much as he does. Boy, is he wrong. When Mxy's not out trying to destroy Superman, he goes home to the 5th dimension where he enjoys long walks in the park, rock climbing and snuggling with his main squeeze Gsptlsnz.

Where and When Superman First Met Him
Superman first met Mxyzptlk when he appeared in Metropolis as the charismatic Ben DeRoy. Mr. Mxyzptlk once forced Superman and The Flash (Wally West) into a race that The Flash won. Mr. Mxyzptlk encountered Lex Luthor and learned the concept of how to lie. Later, Mr Mxyzptlk supplied Luthor with red kryptonite which made Superman's powers disappear, but Luthor violated Mr. Mxyzptlk's conditions and so Superman regained his powers. Mxyzptlk appeared in Metropolis and tried granting a wish to everybody in the city. He even tried, albeit unsuccessful at the time, to reunite Superman and Lois Lane.

There is a time when even a trickster is out-tricked. Mxyzptlk paid a visit to the insane Clown Prince of Crime, The Joker, at his cell in Arkham Asylum. He toyed with the notion of setting the Joker free with 1% of his power and watch Superman and Batman dance to his tune. Smarter than he may have seemed to Mxyzptlk, the Joker tricked Mxyzptlk into revealing his imp name and gained power over him. He took 99% of Mxzyptlk's power and used it to proclaim himself Emperor of a world he reshaped in his image. Superman, finding the flaw in his twisted logic realized that to the Joker, his existence is nothing without Batman. He couldn't even erase his own memories of the Dark Knight and therefore his reality was thwarted and Mxyzptlk was able to reign his powers back in and return to the 5th Dimension

Mxy has done many other things and he got bored of tormenting humans and other beings weaker than him, so when he found Superman, he was thrilled.

Smallville
Mr. Mxyzptlk makes an appearance in the TV show; Smallville, season 4. There are several differences in the Smallville version comparing to the comic book version. For one his real name is Mikhail Mxyzptlk, he is human and is a young teenager, and only have the ability to control minds. He is portrayed by Trent Ford.

Goals and Ambitions
In Superman: The Animated Series, while on trial in his own dimension, Mister Mxyzptlk revealed his disliking of Superman. He explains that he hates Superman, because of all Superman's virtues. His hatred may also stem from the fact that Superman always knows how to defeat him. Therefore, his only goal is to cause as much trouble for Superman as possible.

Trivia

 * Mr. Mxyzptlk is the DC equivalent of the Impossible Man from the Marvel Comics.
 * Mister Mxyzptlk named himself such because he believed Superman would never be able to say or type it backwards (his one original weakness).. other versions have it that his name is the closest he can get to one we humans can ever understand.
 * His infamous weakness is actually self-made and it only works because he allows it to work, in short he functions much like a Q from Star Trek - his weakness is as he decides, however as an Imp he must pick a weakness (this prevents Imps from destroying the multiverse with their tricks).