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.