Bizarrely, Mortal Kombat X has a sizeable slate of classic Mortal Kombat characters that you can only fight against, not play as, Rain and Baraka foremost among them. Clever players, however, have figured out how to use them in the game's PC version.
Player XVermillion tore open Mortal Kombat X's chest cavity and, as hot geysers of ones and zeroes spewed everywhere, found the game's beating heart. There, they crossed a few wires and, boom, playable Rain. They explained:
"There are other files you can change to get stuff like Rain's name under the health Bar 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)."
"Anyway, I backed up the files for Shinnok (the character I'm replacing) and renamed Rain's files to match those associated with Shinnok's 1st costume slot (CHAR_Shinnok_A.xxx and one other, the one that matches Rain's other one) and moved 'em to the folder and overwrote Shinnok's old files with the newly renamed Rain files."
"What you see is just me playing in training mode with what the game thinks is Shinnok but is using Rain's model and moveset."
So basically, they tricked the game into believing they were playing as one character and then swapped their most crucial data for Rain's. Now, this is only easily doable with the game's PC version, as the consoles' figurative lids are screwed on a little tighter. Also, if you do decide to try this at home, probably back up any data you change so as to save yourself from future hassles.
Now, Mortal Kombat X is already guaranteed at least a few DLC characters in the future, so it's possible that Rain, Baraka, and company could show up fully assembled -- not in pieces and with the instructions in computer-ese -- before too long. I mean, it'd be kinda bullshit to charge for them given that they're already mostly in the game, but it could happen. For now, though, this method seems mostly functional, so go nuts.