Successfully complete the game on the Normal difficulty setting to unlock the Hard difficulty setting. You may also start a new game to access a bonus sub-quest and have all previously unlocked costumes, money, and the drill power-up available. Although you will have to find the Headgear again, they will be at their previous levels.
Successfully complete game on the Beginner, Normal, Hard, and Happy difficulty settings. Start new game and get the drill then return to town. You can find Popon in front of Fan's cake shop. Talk to her and you will be able to play as her.
Successfully complete game on the Beginner, Normal, Hard, and Happy difficulty settings. Start a new game and go to the final locked house in human town and enter "LEGENDARY" as a password to unlock Boss Rush mode. Note: Enter "yarushikanaiwane" as a password in the Japanese version in Hiragana/Katakana. Boss Rush mode allows all eight Bosses to be fought in succession.
Movie Viewer
Successfully complete Boss Rush mode to unlock the "Movie Viewer" option.
Original Japanese voice acting
Successfully complete the game, then choose to start a new game. Then at the difficulty selection screen, hold L + R and press Left, Up, Right, Down, Left, Up, Right, Down, Left, Up, Right, Down. If you entered the code correctly, you will hear a sound.
Play as Popon
To unlock extra character Popon, clear game under Beginner, Normal, Hard, and Happy difficulties. Start new game, get the drill, and go back to town. You'll find her standing in front of Fan's cake shop, talk to her and ask her to switch. Popon use sword as weapon, she can't use any headgear or clothes, but she is powerful.
Unlock Armor
Give 30 Gold Medals to Hyperbolic in Beginner mode.
Unlock Chinese Dress
Get the good ending in Hard mode.
Unlock Kimono
Set the system date between January 1st and 7th.
Unlock Leopard Suit
Get the good ending in Crazy mode.
Unlock Maid Outfit
Can be bought from the vending machine at Spinach Caverns - Shadow Grotto.
Unlock Monkey Suite
Get the good ending in Beginner mode.
Unlock Nurse Outfit
Complete Boss Rush mode in Happy mode.
Unlock Pajamas
Get the good ending in Normal mode.
Unlock Princess Dress
Give 30 Gold Medals to Hyperbolic in Normal mode.
Unlock Samurai Outfit
Give 30 Gold Medals to Hyperbolic in Crazy mode.
Unlock Santa Outfit
Set the system date between December 25th and 31st.
Unlock School Uniform
Get the good ending in Happy mode.
Unlock Swimsuit
Complete Boss Rush in Crazy mode.
Unlock Vampire Outfit
Give 30 Gold Medals to Hyperbolic in Hard mode.
Unlock Witch Outfit
Give 30 Gold Medals to Hyperbolic in Happy mode.
Unlock Work Uniform
Complete Boss Rush mode in Hard mode.
Unlock Workout Gear
Complete Boss Rush mode in Beginner or Normal mode.