Play As Human Smoke:

Beat the 2nd tower (Warrior) and when you start another 1p game you will be able to choose Human Smoke at the Mystery Box. Note: You can only choose Human Smoke once in the mystery box.

Play As Motaro:

Beat the 3rd tower (Master) and when you start another 1p game you will be able to choose Motaro at the Mystery Box. Note: You can only choose Motaro once in the mystery box.

Play As Shao Kahn:

Beat the 4th tower (Grand Master) and when you start another 1p game you will be able to choose Shao Kahn at the Mystery Box. Note: You can only choose Shao Kahn once in the mystery box