Who is Mxyzptlk?
Mister Mxyzptlk is an impish character who appears in Superman comic books. He is a sorcerer from the fifth dimensions who has a penchant for playing pranks on Superman.
His first appearance was in Superman #30 (Sept. 1944) in the story “The Mysterious Mr. Mxyztplk”. He is portrayed as a small bald man typically wearing a bowler hat.
His powers in our dimension are nearly limitless. He can only be stopped by tricking him into saying or spelling his own name backwards. Doing this will return him to his home in the fifth dimension. Once he is returned he must stay for a minimum of 90 days. He also cannot stand being ridiculed.
Mxyzptlk on Supergirl
It has been announced that the Supergirl TV series on the CW will be bringing the character into the show. Peter Gadiot has been cast as the mischievous imp. It seems that Mxysptlk has a fondness for Kryptonians other than Superman. It has not been said whether the character will resemble the comic book version or is they will re-imagine him more normal in appearance to help alleviate special effects costs.