Happy Birthday: Mr. Mxyzptlk
An imp from the 5th dimension, Mr. Mxyztplk is an inveterate troublemaker and jester. Because he’s from another dimension, Mxyzptlk is not bound by our universe’s laws and can produce a variety of magical effects with a single thought.
Shortly after arriving on our world, Mxyzptlk encountered Superman, and the the imp has delighted in tormenting the Man of Steel ever since that first meeting. The only way to get rid of Mxyzptlk is to trick him into saying his own name backwards—this sends the imp back to the 5th dimension for at least 90 days.
It never lasts long, though, and soon Mxyztplk is back to tease and confuse Superman yet again.