Secret Fighters
If you beat The Champ and then successfully defend your title five consecutive times in each different fighting style, you'll reveal six new characters. They are as follows:

Peek-a-boo - Kiki & Mimi, Edward King.
Detroit - Roboxer Beta 1, Carrie the Bunny.
Open - Jack-in-the-Box, Snake.

Extra Heads
Whilst competing in the main event, once you have fought and beaten Takeshi Hangman, The Champ and Alan Kidd, return back to the 'create you own boxer' mode (whilst with the same gym you need to beat them), and then highlight the 'head' option. Now scroll off the list available by either going left or right, and then the heads of the three boxers we just mentioned will be available for you to select.