Start a new boxer in the Main Event. Pick one of the boxers and imediately save him to your memory card. Exit, then repeat with another character. When you have saved each boxer start a two-player match. At the character select screen press SQUARE to pick one of the other boxers from your memory card.
Play as "Jackel"
To play as Jackel you first have to go to the "Main Event" and save any player. Then go back into "Exhibition" and load the player you saved by pressing SQUARE. After you highlight the player press SQUARE again and you should have Jackel in your second "Mystery Box".
Quick Recovery
When you get knocked down rotate the D-Pad and press other buttons repeatedly to get up faster.
