I took a number of useful or just plain cool cheats/secrets from the handy dandy GameFAQs! Of course, you shouldn't use any of these in your first play through. If you know of more, I'd love to add them, as long as they're tested to work. Have fun!

Emeth is that weird, earth golem in the alchemy room you discover early on in the game. Apart from his name on the plate to gain access through the door, this cheat will trigger an alternate cutscene and give you a new item.
METH ~ gives you an obsidian choker

You must have beaten the game once to use this. Use these plate keys to get items you can equip, from the machine in the alchemy room.
ADAMAS ~ gives you a diamond choker
ACLHYMIA ~ gives you a pair of alchemist earrings
POWDER ~ gives you a pair of burst shoes
MORGAN ~ gives you a pair of fairy earrings
MAGNUS ~ gives you a pair of inquisitor earrings & jade choker

Invisible Lorenzo When you first arrive at the House of Truth, go to the hallway outside Lorenzo’s Room but don’t enter his room yet. Go down the spiral staircase and collect the Golden Candlestick, then return to the 2 vases at the hallway. Kick the vase on right and a Luminescent will be free. Guild the Luminescent back to the West Garden and go down to the area where the explosive is placed. Stand on the platform under the lamp and a cutscene will play.

If you do it correctly, Lorenzo will be invisible in the cutscene. You can also bypass the old Lorenzo’s boss battle and the scene where you got trap in a room. Lorenzo will remain invisible and never chase you but this glitch will last up to the final boss battle, unless you get yourself trapped in the room. Because Lorenzo will return after the cutscene where he turns young.

There are a large number of skimpy costumes our heroine can wear in replays! Here is how you get them:

Illegal in Some States ~ Finish the game once with either Ending A or Ending B
Cowgirl ~ Finish the game once with Ending A
Frog ~ Finish the game with Ending C
Stuffed Dog Costume (Hewie) ~ Finish the game on Hard Mode
Alternate Enemy Costume ~ Finish the game on Hard Mode
Alternate Dog ~ Finish the game--and that's it!

Easter Eggs
Towards the end of the 2nd chapter of this game (when the maid is chasing you), players will descend into a crypt where a Golem, tomb and plate machine can be found. Create a plate labeled SALTATIO and insert it into the golem for a brief, 10 second Para Para dance sequence.