Take the Crystal (found in Room 99) to Room A0. At the final boss (Room B3), fire at him until he is encased in a bubble. Now, instead of popping it right away, first hit Start to summon Player 2 (you must have at least one life left). Now pop the bubble for the ''good'' ending.
Enter at the screen where you select which game mode you want to play. There is a password field there.
If you die in a two-player game, hit Start to pause the game, then hit Select to steal a life from your partner.
Enter these passwords in the same place you normally enter round passwords on the mode select screen.
The game has 4 endings, depending on the following: