There's a number of characters in Mortal Kombat X that players aren't supposed to be able to fight with and against, but you can on the PC if you tweak some of the file names in the game's data folders.
For example, you can play as Rain, as shown in the video below.Redditor XVermillion for the tip:
Doing so is very easy too. All you have to do is rename the files for Rain's data the same as one of the legitimate characters in MK, perhaps one you don't use very often (whilst remembering to back them up in-case you want to bring them back later). This allows you to use that slow for Rain, or similar, choosing them and playing as if they were a costume of a character you didn't otherwise like.
"There are other files you can change to get stuff like Rain's name under the health Bar," said Xvermillion, "but the only files I modded are in the "Assets" folder I believe; there should be 2 associated with Rain (Rain_A.xxx and another one I can't remember but you can just search the folder for Rain)."
Chances are the publisher was planning on selling Rain DLC down the road. I wonder if that will still happen at this point?