The only way you can use the codes below is by finishing both levels with a five star rank under hardcore difficulty to unlock them. Every code has two parts, each of which are unlocked by a specific level (for example, at the end of level 1 and level 2 with at least a five star rank) on the game art that appears

Helium Hunters:
In the main menu, push R1(2), Triangle, Circle, Square, L2, L1, Down. Finish the Strapped for Cash and View Of Innocence levels to unlock this code.

Monkey Skin:
In the main menu, push Square(2), R2, Down, Triangle, Square, Circle, Down. Finish the Press Coverage and Wrong Side Of The Tracks levels to unlock this code.

Piggsy Skin:
In the main menu, push Up, Down, Left(2), R1, R2, L1(2). Finish the Key Personnel and Deliverance levels to unlock this code.

Rabbit Skin:
In the main menu, push Left, R1(2), Triangle, R1(2), Square, L1. Finish the Kill The Rabbit and Divided They Fall levels to unlock this code.

Regenerate:
In the main menu, push R2, Right, Circle, R2, L2, Down, Circle, Left. Finish the Fueled By Hate and Grounds For Assault levels to unlock this code.

Runner:
In the main menu, push R2(2), L1, R2, Left, Right, Left, Right. Finish the Born Again and Doorway Into Hell levels to unlock this code. This code allows unlimited sprint.

Silence:
In the main menu, push R1, L1, R2, L1, Right, Left(3). Finish the Road To Ruin and White Trash levels to unlock this code.

Super Punch:
In the main menu, push L1, Triangle(3), Circle(3), R1. Finish the Mouth Of Madness and Doing Time levels to unlock this code.

Alternate title screen:
Finish the game, then press L1 + L2 + R1 + R2 at the title screen to hear a backwards message being whispered. Note: A new segment of whispering can be heard each time the buttons are pressed