You're not meant to be able to play as Rain in Mortal Kombat X, but thanks to the determination of the PC community you now can. As the video above demonstrates, it's possible to play as non-playable characters in Mortal Kombat X, so long as you're prepared to tweak a couple of filenames. As always, you should back up everything before you do so.
According to Reddit user XVermillion, who is responsible for both the mod and the video above, it's pretty easy. "I backed up the files for Shinnok (the character I'm replacing) and renamed Rain's files to match those associated with Shinnok's first 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."
XVermillion has more instructions over here if you want to try it out. If Warner Bros. Interactive intended to sell Rain as DLC down the track, then... sorry about that, Warner Bros. Interactive.