Mr. Mxyzptlk

"But I have yet to begin to shenanigan! And I shall shenanigan again and again!" - Mxyzptlk

Background
Mr. Mxyzptlk (prounounced 'mix-yez-pittle-lick') is an Imp from the 5th Dimension. He (and others of his race) have visited this universe in the 3rd dimension before, giving rise to a lot of stories of genies, spirits, and (of course) imps. He also might have been 'Puck' from a Midsummer Night's Dream. He wasn't always the prankster he is today, though. His early visits to Earth were as a serious student, studying the lower dimensional beings. His studies of the videotapes of the comedy troupe 'The Three Stooges' changed his mind forever, turning him into the annoying prankster he is today.

Despite immense power, Mxyzptlk is a slave to two things - irrational compulsion towards practical jokes, and his own self-imposed rules (which are possibly enforced imposed by the collective society of Imps of the 5th Dimension, called Zrfff). One of the rules is that, while they can do anything to anyone in the 3rd dimension, it would only be permitted without severe punishment if it can be considered a prank. Also, for some reason, they all have an intense fear of vowels that is never, ever explained - except that when they use names that are pronounceable to lower dimensional beings, they make sure that there are no vowels involved. Ever. Well, except 'y.'

Another rule is that any imp that makes a rule or rules about himself or herself, as part of the game played with the lower dimensional beings bounds him to that rule. In Mxy's case, in order to make his tormenting of Superman fun and give the Kryptonian a chance to win (because what fun is there if there's no sport to it?), if Mxyzptlk is tricked into saying, writing, spelling, or in any other way expressing his name backwards - Kltpzyxm - he is banished back to the 5th dimension for a period of 90 days. He sometimes has altered this rule a bit (more difficult rules for greater stakes, for example), but can never make the rule 'weaker.'

He sorta wishes he didn't make that rule sometimes, and definitely wishes he didnt make it a permanent rule, because despite being vastly powerful and nearly omnipotent, he tends to get easily tricked. Repetitively.

Very repetitively.

Personality
Mxyzptlk is not actually 'malicious.' In fact, sometimes his visits are harmless. Sometimes dangerous. Always annoying.

Despite his vast power, Mxy's not actually interested in harming people or killing his enemies - just making them look foolish for his amusement. He rarely takes fighting seriously and normally spends his battles goofing off. He also will always abide by his own rules as a matter of course. If he makes a one time rule, for example, that if his face is painted blue that he will go back to the 5th Dimension for a year, his own powers will force him to go back for an entire year.

Whenever he's banished, all the damage and effects that he cause are reversed or fade - showing that, while dangerous and mischievous, he's not actually 'evil' per se.

Just a prankish, goofy jerk.