Items Trick
At anytime during the game, check your stats and press R2,L1,R1,L2,R2,L1,R1,L2,L2. This trick will let you use the items you normally use on monsters. You can use them to increase your HP,Strength,Defense,etc.
Submitted by Fredricktt
See Ending
At the Title Screen, press L2,R1,L1,R2,L2,R1,L1,R2,R2.
Submitted by Fredricktt
Enable Monsters
At the shop where Monsters are sold, talk to the cashier, press R2,R1,L2,L1,R1,R2,L1,L2,L2. You will then be able to buy any monster in the game, including the characters.