Alternate costumes 
At the character selection screen, press Circle + Square. 

Champ class 
Select "Championship Mode" from the main menu. Then, choose 
"New Game" and enter "CHAMP" as a gym name to unlock all class 
champ boxers in championship mode. 

This also unlocks Damien Black in arcade mode. 

Gold class 
Select "Championship Mode" from the main menu. Then, choose 
"New Game" and enter "GOLD" as a gym name to unlock all class 
gold boxers in championship mode. 

Silver class 
Select "Championship Mode" from the main menu. Then, choose 
"New Game" and enter "SILVER" as a gym name to unlock all 
class silver boxers in championship mode. 

This also unlocks Bruce Blade in arcade mode. 

This also unlocks Nat Daddy in arcade mode. 
Bronze class 
Select "Championship Mode" from the main menu. Then, choose 
"New Game" and enter "BRONZE" as a gym name to unlock all class 
bronze boxers in championship mode. 

This also unlocks Kemo Claw in arcade mode.