Load up the game and highlight continue. Input Triangle, Square, Circle, Triangle, Square, Circle, X. If you do this right, then you will hear Maos voice. After this, start the game normally.You'll now find the Parallel Worlder at the bottom levels of the base under the stairs. After clearing the tutorial, talk to him and the new content will become available to you.
Complete the indicated task to unlock the corresponding cameo character.
To unlock the following DLC characters, finish the main storyline, then complete the indicated task.
Each of the Diez Gentlemen can be acquired by starting a New Game+ while Mao is at at a certain level or above
Unlock the Postgame, then talk to the Angel NPC by the Dimension Guide.