Play As Kappah:
Enter arcade mode & go to Gore & hold Select, go to Hayato & press Circle, Square, Triangle, Square, X, Square, Triangle, Square, Circle, Square, X + Triangle.

Play As Blood:
Enter arcade mode, hold Select & go to Bilstein, press X, Square, X, Square, X, Square, go to Kappah & press Circle, Triangle, Circle, Triangle, Circle, Triangle, then press L1 + R1 & let go of Select.
(You can save these into the memory card.)

Dark Fighting:
Choose your character & hold Down + L2 + R2 until the match starts.

Big Heads:
After you choose your character, immediately hold Right + Start + Circle + Square until the match starts.

Small Heads:
After you choose your character, immediately hold Left + Start + Circle + Square until the match starts.

Invisible Walls:
If you beat the game on level 4 difficulty or above, you'll find a Wall option in the options menu. If you turn it on, an invisible wall will surround the arena.