A Mortal Kombat X modder has had a dig around the PC version's files and made currently unplayable characters playable.
Modder XVermillion published a couple of videos to YouTube, embedded below, showing Rain, Baraka and Sindel in training mode.
You fight these characters during MKX's story, but they are not playable characters - yet. Perhaps they'll be sold as downloadable content at some point in the future.
XVermillion explained his working out in a post on Reddit. Essentially, he tweaked playable character Shinnok to use non-playable character models and movesets.
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.