Beat the game four times to unlock the four hidden characters.

At the title screen, press Select 10 times. If you entered the code correctly, you will hear a chime.

At the character selection screen, highlight a fighter. Then, hold Down + X + Circle and press Start.

At the character selection screen, highlight a fighter. Then, hold Circle + Square and press Start.

At the title screen, press Left 10 times. If you entered the code correctly, you will hear a chime.

At the title screen, press Right 10 times. If you entered the code correctly, you will hear a chime.