Enter the following codes at the "Press Start" screen. A message will appear to confirm correct code entry. 

Unlimited Health:

Hold L1 + L2 + R1 + R2 and press Up, Right, Down, Left, Up, Left, Down, Right 

Unlimited Combo Meter:

Hold L1 + L2 + R1 + R2 and press Left, Right, Up, Down, Right, Left, Down, Up 

All Upgrades:

Hold L1 + L2 + R1 + R2 and press Up, Up, Left, Left, Right, Right, Down, Down 

All Bonuses:

Hold L1 + L2 + R1 + R2 and press Down, Up, Down, Up, Left, Right, Left, Right 

Dark Knight Difficulty Setting:

Hold L1 + L2 + R1 + R2 and press Down, Left, Right, Up, Up, Right, Left, Down 

Unlock the Making Of Sin Tzu:

Defeat Sin Tzu at the end of the game. 

Alternate Costume:

Successfully complete the game under the easy difficulty setting.

Thanks to RamirezXsJ