Alternate costumes:
Press Start or Select when choosing a fighter at the character selection screen.

Fight as Dizzy or Testament:
Press Down, Right(2), Up, Start at the "Press Start" screen.

Dizzy:
Defeat the Dizzy's dare devil version in survival mode. Alternately, defeat Dizzy on stage 10 in arcade mode.

Testament:
Defeat Testament's dare devil version in survival mode. Alternately, defeat Testament in stage 9 and Dizzy on stage 10 in arcade mode.

Extra mode:
By beating a fighter's dare devil version in survival mode you will unlock extra mode for that fighter. The fighter's extra version is able to execute new moves and supers.

Original mode:
After finishing the game in survival mode you will unlock the "Original Mode" selection at the options screen.