Go to the Menu screen at the Soulin Monster Ranch and highlight "Buying A Monster." Enter R2, R1, L2, L1, R1, R2, L1, L2, L2. You'll hear a confirming sound to let you know it worked. Now you can select from a plethora of different monsters

At the Title/Main Menu screen enter L2, R1, L1, R2, L2, R1, L1, R2, R2. You'll hear a sound to confirm that the code worked. A new menu option called "Ending" will appear underneath the others. Accessing it will allow you to get the ending of the game.

At the options screen, press R2, L1, R1, L2, R2, L1, R1, L2, L2