Final Bout
The following codes work on the Japanese version.
Unlock hidden characters: At the title screen, hit right, left, down, up, right, left, down, up. If done correctly, you should hear a chime. Now you will have 6 extra characters at the character select screen.

Unlock Super Saiya-Jin 4 Son Goku: Do the previous code, but press triangle 5 times and X 9 times after the first chime. You should hear the same sound again. Now you can play as Super Saiya-Jin 4 Son Goku.

Play as a wireframe: After you select your character, immediately press Select. Continue holding Select until the fight starts. You're character will be replaced by a wire frame of that same fighter.