At the Title Screen press 1, Right, Left, Down, Up, 1, 1, 1, 1, Up, Down.
1, Right, Left, Down, Up, 1(4x), Up, Down at the Title Screen.
Enter the following password on the password screen.
This lets you enter a special VS mode against the computer in which you can choose absolutely any character in the game, any enemy. You can also choose the level of your character(A to Z) and the background.