Enter CHAMP as the gym name in championship mode and you will get all the boxers in Championship and Arcade mode.
Alternate costumes
Press Square + Circle at the character selection screen.
Bronze class boxers
Enter championship mode and enter BRONZE as a gym name. Note: This also unlocks Kemo Claw in arcade mode.
Energy Regain
When you get knocked down in a fight, hit the buttons rapidly - X, Square, Triangle and Circle. this will regain your energy. (note: you can also hit the buttons X, Square, Triangle and Circle when you knock someone down, and that will also regain your energy.
Fighting Tips - Turbo Mode/Rumble Mode
When you get "rumble" hit and hold R1 and L1 at the same time, then you will get the tourbo mode which gives you infinite stamina.
Gold class boxers
Enter championship mode and enter GOLD as a gym name. Note: This also unlocks Nat Daddy in arcade mode.
Selene Strike move
Press Forward, Forward, Triangle, Back, Back, X to throw a spear.
Silver class boxers
Enter championship mode and enter SILVER as a gym name. Note: This also unlocks Bruce Blade in arcade mode.