Mortal Kombat X features several characters like Rain and Baraka that aren't actually part of the playable roster. However, players have found a way to get around this restriction.
In the below YouTube video, a player is using Rain in a practice match against Sub-Zero. As XVermillion explained on Reddit, he managed this feat by editing the files of the PC version:
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)
You can see the fruits of his efforts here:
The trick isn't just limited to Rain. The same player also released a video showing him play as Baraka and Sindel. Again, he did it by swapping in the assets for those characters:
It's not a perfect solution. As he notes, Sindel's flight and hair whip weren't working. There also doesn't seem to be a way to replicate these exploits on the console versions of the game. However, this is currently the only way that MKX players can take on the role of these classic fighters.
Why would Netherrealm include these characters in Mortal Kombat X but not add them to the playable roster? It's possible that they're planning to release them as DLC as a later date. If so, we may have to wait awhile for these fighters as Netherrealm still needs to release the four characters in Mortal Kombat X's Kombat Pack.
I doubt anyone would be surprised to see more microtransactions out of the game, though. MKX launched with several DLC packs on day one. Players can buy additional skins, Goro, and even easy Fatalities if they're willing to pay a bit more.
MKX is now available on PC, PS4, Xbox One, Xbox 360 and PS3. If you need help mastering the game, check out our guide to every character's Fatalities.