Instructions
Go into "Party Mode" from the main menu. This is a mode where you will have to fight 99 computer-generated opponents back-to-back. You only have a limited amount of life energy and time for each match.
Choose the character you wish to use in this mode. Choose one that you know well, as this mode is extremely difficult and you will need to be at the top of your game.
Unlock every hidden striker in the game. Strikers are unlocked after defeating different numbers of opponents. Defeat 14 to unlock Heavy D, Lucky and Brian, 27 for Blood Lori and Blood Leona, 36 for Wolfgang Krauser and Mr. Big, 40 for Hiedern, 70 for Blood Orochi and 99 for Krizald. These are all the strikers; once they're all unlocked a message in the game will tell you how to unlock Zero.
Press up and circle simultaneously on your controller on the "Order Select" screen and Zero will become available to you.