Up, Down, Left, and Right refer to the D-pad.

L3 and R3 refer to the left and right joystick buttons.

1. Unlock All Characters.
At the title screen, input:
L1, R2, L2, R1, Right, Left, L3, R3.

2. Unlock Stronger Story Mode Characters.
At the title screen, while holding L1 and R1, input:
Left, Left, Right, Right, Up, Up, Up, Up.

3. Unlock All Missions.
At the mission select screen, input:
L1, R1, L2, R2, Right, Square, L3, R3.

4. Unlock All Mission Layouts.
At the mission select screen, input:
R3, L3, R2, L2, R1, L1.

5. Unlock All Items.
At the item select screen, while holding R1 and L1, input: 
Up, Square, Square, Left, Square, Square, Down, Square, Square, Right, Square, Square.

6. Unlimited Item Capacity.
At the item select screen, while holding L1, L2, R1 and R2, input:
Square, Square, Square, Up, Left, Down, Right, Square, Up, Right, Down, Left.

7. Increase Items.
At the item select screen, while holding R2 and L2, input:
Square, Square, Square, Up, Left, Down, Right.

8. Show Time & Score.
While playing, pause the game, then with the second player's controller, input:
Right, Right, Left, Left.

9. Increase Score (+100).
While playing, pause the game, then while holding L1 and R1 with the second player's controller, input:
Right, Right, Left, Left.

10. Turn Special Abilities On/Off.
While playing, pause the game, then while holding L1 and L2, input:
Up, Up, Down, Down, Release L1 and L2, Square, Square, R1, R2.

11. Increase Kanji.
While playing, with the second player's controller, input:
Up, Up, Down, Down, Square, Square, Square.

12. Control Nearest Enemy.
While playing, pause the game, then with the second player's controller, input:
L1, R3, R1, L3, Right, Square, Up, Square, L2, Left, Square, Down, R2.

13. Recover Health.
While playing, pause the game and input:
Up, Down, Right, Left, Square, Square, Square.

14. Unlock "B-Side" Audio.
At the title screen, while holding L1 and R2, input:
Down, Square, Up, Square, Right, Square, Left, Square.

15. Unlock "Through The Portal" Stage.
At the title screen, input:
L1, Up, R1, Down, L2, Right, R2, Left.

16. Play Demo Level.
At the title screen, input:
Up, Down, Right, Left, X, X, X.

17. Reset Game.
While playing, hold:
R1, R2, L1, L2, Start, Select.