While choosing a character, press Left, Right, Left, Left, Right, Up, Down, R1 (five times) and R2 to unlock everything. A sound should confirm the code. Use Square to change between costumes.
Every time you beat the game a new boxer will be unlocked such as Michael Jackson and Freke E. Deke.
While choosing a character enter Right, Right Up, Down, Right, R1, R1, R2.
Same as above but enter Right, Right, Up, Down, Right, R1, R2.
Same as above but enter Left, Up, Right, Down, R1, R1 and R2.
Same as above but enter Left, Right, Up, Down, R1 and R2.