Configuration mode:
Start match and finish the fight. Hold R + X + A at the continue screen until the next match starts. The configuration mode screen will appear.

Hidden options mode:
Start a one player game, and press X + A + L when the fighter's names are displayed.

Fight against the CPU in Duel and Team modes:
Hold L + R on the character selection screen to control the other player's selection. Hold L + R and press Y to select your opponent's character. That character will be CPU controlled at the current difficulty setting.

Bosses:
Highlight the "Story Mode" option, then hold L + R and press Start.

Fight with Happy and Colon:
Press Select + Start + L + R to get a secret level and be able to fight with Happy and Colon.

Debug mode:
Hold L + R and press Start at the title screen. Continue to hold L + R until the debug options appear. Matches may now be set up using any character against a player or the CPU. Note: The CPU can not fight as Colon or Happy. Another option allows character strength to be adjusted. Press Select to change the background. Press Start to begin a match with the current settings.

Change character:
Hold L + R and press Left or Right at the character selection screen to select any character except for Happy or Colon.

Fight as same character:
Select the same character, then hold L + R on controller one while pressing any button to start the match.

Additional difficulty levels:
Press L, R, Select at the options screen. Two more difficulty levels will appear in red text.