All the following codes are entered at the main menu screen:

Play as Kemo Claw
Press left, left, left, right, right, right, left, right, left, right. A confirmation tone should play if you input the code correctly.

Play as Nat Daddy
Press right, right, right, left, left, left, right, left, right, left. A confirmation tone should play if you input the code correctly.

Play as Damien Black
Press right, left, right, right, left, left, right, right, right, left, left, left. A confirmation tone should play if you input the code correctly.