The video above is the Mortal Kombat X How to Use Unplayable Characters Guide and shows how to play with Baraka, Rain, Sindel and Corrupted Shinnok in Mortal Kombat X.
Note that the method works only on the PC version of the game.
To use an unplayable character you must first go to the Asset folder of the game and backup the files of an existing playable character.
For example, the files for Shinnok are CHAR_Shinnok_A.xxx and Char_Shinnok_A_ScriptAssets.xxx.
After that you need to copy the files of an unplayable character and rename them to match the files of the backed up character.
For example, the files for Baraka are Baraka_A.xxx andBARAKA_A_SCRIPTASSETS.xxx.
While in game you need to select the character that you replaced, even if it shows the original character or no character at all.