The character will appear in two episodes. No official character bio has been released, and no actor has been cast, but The CW confirmed Mxyzptlk's appearance for ComicBook.com.
Mr. Mxyzptlk is a powerful "fifth-dimensional imp" whose battles with the Man of Steel are defined by a sense of fun and mischief more than malice -- but that doesn't mean there won't be destruction when he comes to town. Mxyzptlk's idea of a good time is to make a building come to life and start walking around town, crushing everything in its wake while it throws a temper tantrum.
In most versions of the mythology, Mr. Mxyzptlk can only come to the DC Universe every so often; when he's sent away, usually by being tricked into saying or writing his name backwards, he returns to his own dimension and comes back only once the "rules" allow him to.
There are some stories that have reinvented him as a more "serious" threat with darker understones, including Countdown to Final Crisis and the Grant Morrison run on Action Comics which came at the beginning of DC Comics's 2011 contunity reboot, The New 52. It seems unlikely that a show as light as Supergirl, though, would take one of comics's most lighthearted villains and make him anything too dark.
Created in 1944 by writer Jerry Siegel and artist Ira Yarborough, Mr. Mxyzptlk features heavily in the recently-reprinted World's Funnest collection, in which he faces off with his fellow god-like, childlike imp, Bat-Mite. Supergirl will not be the character's live-action debut; he previously appeared in The Adventures of Superboy, Lois & Clark: The New Adventures of Superman, and Smallville.
Yesterday, EW reported that, with the Invasion! crossover now in the rear-view mirror and the midseason finales for The Flash, Arrow, and DC's Legends of Tomorrow set to air next week, producers' attention was turning to prepping the Supergirl/The Flash musical crossover episode. While there's certainly no guarantee Mxy will serve as an antagonist for such a story, there is a certain logic to it: in one memorable post-Crisis on Infinite Earths story, Mxy entertained himself by forcing Superman and The Flash to have a race to prove who's fastest, while creating destructive diversions all around them.
Supergirl airs Monday nights at 8 p.m. ET/PT on The CW. New episodes will return in January. There is no known date yet for the episodes which will feature Mr. Mxyzptlk.
Additional reporting by Jay Jayson.