Win bout:
Hold L1 + L2 + R1 + R2 and press X during a bout in road to world title or tournament mode. Press X again to display a loading screen instead of the game trivia. Press X again to win the current bout.

Secret boxers:
In Open Style, fight until you are champion to access a Jack-In-The-Box boxer. Defend the championship title five times to access Snake.

In Peek-A-Boo, fight until you are champion to access Kiki and Mimi. Defend the championship title five times to access Edward King (the promoter).

In Peek-A-Boo, fight until you are champion to access Roboxer. Defend the championship title five times to access Carrie (the girl that carries the round cards).

More custom heads:
Compete in main event mode and defeat Takeshi Hangman, The Champ and Alan Kidd. Return to create your own boxer mode and stay in the gym that was used to defeat the previous boxers. Move left or right off of the available list to find the heads of those three boxers.