Cheats:
Easy mode
Beat Normal Mode.
Hard mode
Beat Normal Mode.
Very hard difficulty
Beat the game in Hard and there will be this new difficulty.
Weapon upgrade discount
First, you must buy the Stuffed Doll, Cushion, and Model Ship from the General Store. These items are not known to have any particular purpose, or at least so described by the game, so buying them normally seems pointless. However, you can give them to Roll and you will get a 10% discount on any weapon upgrade you make from here on out.