Real Bout Fatal Fury Special: Shadow characters
At the character selection screen, move to the fighter you want to play as then do the following. Hold Start and press B(2), C(2) then, hold B for about a second. While still holding Start + B, hold C. While holding those three buttons, press A or D to select your character in Shadow form. You may now release all buttons. A or D is used to select the color of your character's clothes.