Originally meant to be a joke character based on song Purple Rain by Prince, Rain was eventually implemented as a real character in Ultimate Mortal Kombat 3. He's appeared in just about every Mortal Kombat game since his initial appearance, and now as the latest DLC character for MK9, he returns again.
I've gotta say, for a dude that uses rainwater as his primary method of attack, he's got some pretty killer moves in his repertoire. I mean, did you see that one fatality? He broke that dude's legs! With an orb of rainwater!