Skip Level

During gameplay HOLD R1 + R2 + L1 + Select and press X, Circle, Square, Triangle, X, Circle, Square

Unlock two more characters in arena mode!

Start or continue a campaign. Beat vadridgar(i think that's his name)the arena master for the second time. Then win the next match and you will have two new characters in arena mode! 

Unlock Extra Characters/levels

Beat the campaign mode on the "I can win" difficulty and after the credits a new level is playable against two new characters. Beat this level and you can play it and the characters in the multiplayer arena. (note: you have to load, then quit your campaign after unlocking this level once you turn off your PS2)  

Find the KAMIKAZE!

If you haven't yet found the KAMIKAZE then listen up. On the level "the forgotten place" there is a patch in wall that is boarded up a little bit, it looks like someone has blown a hole in it and someone else had patched it up. Use any gun (except the "gauntlet") and shoot the wall to reveal a hole then walk inside. To you're left should be the Kamikaze, pick it up and press square to obliterate yourself and your opponents!

Unlimited Quad Damage

When you fist start enter your name as "HAT_TRICK" and you will have unlimited Quad Damage through the whole game!