Bonus Wrestlers
Complete all three game modes to unlock an option to play as itchy & Scratchy for one level, and Kang for another and Smithers for another
Cheat Mode
Press Circle, Up(2), Down(2), Left, Right, Left, Right at the "Press Start" screen. This should unlock a "Bonus Match Up" at the main menu. This option allows you to play as any character and select any arena. This also can be unlocked by beating the new challenger, defender, and champion circuits.
Bumblebee Man
Play the New Challenger circuit mode, and finally you'll fight Bumblebee man, after you defeat him, save the game to unlock him as a playable character
Ned Flanders
Play the champion circuit mode, and finally you'll fight Ned Flanders, after you defeat him, save the game to unlock him as a playable character
Moe
Play the defender circuit mode, and finally you'll fight Moe, after you defeat him, save the game to unlock him as a playable character
Professor Frink
Play the champion circuit mode, and finally you'll fight Professor Frink, after you defeat him, save the game to unlock him as a playable character
