Enter the following codes while playing....

Invincibility:
L1, R1, L1, R1, Up, Down, Left, Right

Unlimited Weapons:
L1, R1, L1, R1, Up, Down, Left, Down

Drop Pod:
L1, R1, L1, R1, L1, Up, Down, Left, Up

Quad Damage:
R1, R2, R1, R2, Up, Down, Up, Down

To play as Mr. Phatt:
While fighting Mr. Phatt, finish him off with a combo that does at least 48 points of damage. Once he's unlocked, you can select Mr. Phatt on the Character Selection screen.

To play as Abaddon:
Complete the entire game in Captain difficulty mode. Once he's unlocked, you can select Abaddon on the Character Selection screen.

To play as Agent 326:
Beat the Cephalapods (end of game bosses) with a final score of at least 1,000,000 points.

To play as Overseer:
Beat the game with all of the NON-hidden characters - make sure to save after you win with each guy.