Unlock Alternate Dog:
Finish the game once.

Unlock Alternate Enemy Costume:
Play hard mode.

Unlock Cowboy Costume:
Finish the game once.

Keyplate passwords:

ADAMAS: Rank 02 Treasure Hunter; Diamond Choker (second playthrough)
ALCHYMIA: Rank 01 Top Breeder; Alchemist's Earring (second playthrough)
EMETH: Move the Golem away from door
MAGNUS: Unlock red box in nursery room
METH: Destroy Golem and gives you a choker
MORGAN: Rank 04 Track Star; Fairy Earrings (second playthrough)
POWDER: Rank 03 Warmonger; Burst Boots (second playthrough)
POWDER: Set of Burst Boots in the Alchemy Room slot machine
REST: Access the rest room
RRL, RRRR, RRR, RRLRR: Move Golem in graveyard cave
SALT, SULFUR, MERCURY: Three-headed snake statue puzzle
SALTATIO: Earth Golem Para Para dance and restores your stamina