How do you unlock all characters in Bullet Bill 2

To unlock all characters in Bullet Bill 2, you need to go to the Character Selection Screen and enter the following passwords:
Helicopter-7H0MP
Super Koopa-6E7KU
Chain Chomp-8T9HF
Grinder-3Q7YX
"Flaming Fly Guy"-6F4JW
Gold Shell-9C8PK
Unnamed character (he appears in the game's intro)-9P1WL
Banzai Bill-8E7UC