Secret Options Menu
While holding L at the character select menu, input the following: 

C-Up, C-Right, C-Left, C-Down, B, A 

Then go to the option menu and you will see a new menu for Secret Options. 
Now you can choose itty bitty/massive bodies , high/low voice, background 
select, and a few other options! 



Play as Booger Man
Holding down the left shoulder button (L) at the character selection screen, 
move joypad Up, Right, Down, Left, Right, Left. 

Play as Dr. Kiln
If you beat the game on psycho, you receive the following code: 

Holding down the left shoulder button (L) at the character selection screen, 
hit: B, C-Left, C-Up, C-Right, C-Down, A. 

Play as Sumo Santa
Holding down the left shoulder button (L) at the character selection screen, 
hit: A, C-Down, C-Right, C-Up, C-Left, B. 

Random Character Selection
Hold down the 'R' and 'L' buttons at the character selection screen and a 
random fighter will be chosen.