Start a mission, then pause the game and hold down down L1, L2, R1, and R2. Now tap in the following codes:
Note: You may have to beat the game to acces codes with a "*".
*All Upgrades: Tri, Cir, Tri, Cir
*Invulnerable: Tri, Squ, X, Cir
*Slow Motion: Tri, Cir, X, Squ
*Always Devastating: Squ, Squ, Cir, Cir
Twitchy Enemies: Tri, Tri, X, X
*Infinite Missiles: Squ, Cir, X, Tri
Refill Health: Tri, Down, X, Up
1000 Exp: X, Down, Down, Down
*All Skills: Tri, Cir, Tri, Cir
Level 2 Skills: Cir, Right, Cir, Right
Level 3 Skills: Tri, Up, Tri, Up
Level 4 Skills: Squ, Left, Squ, Left
Level 5 Skills: X, X, Down, Down
*Mini-Enemies: Tri, Tri, X, X
Restore Missiles: X, Down, Tri, Up
Also, beat the game with all three characters to unlock Insildur.