How to unlock Rivals in Tetris Attack
Steps
The "rival" characters can be accessed and used as playable characters, but only in the 2-P vs. mode.
To unlock the four secret characters:
- Press both L and R simultaneously on both controllers on the character-select screen
- Press A on controller number 2.
Pressing L and R on both controllers will cause the rivals to appear on the bottom of the screen.
Pressing A on controller number 2 will cause Player 2 to select one of the rivals. If Player 2 does not wish to select a rival, he may press B.
Source:
Tetris Attack for Super Nintendo Entertainment System