Alternate costumes
When selecting a character, press [L] (clone costume of your enemy if you are fighting yourself) or [R] (different costume).
Fight as Geese
Successfully complete the game with the Fatal Fury team.
Fight as Iori
Successfully complete the game with the Hero Team.
Extra Strikers
Successfully complete the game with any team to unlock K', Maxima and Vanessa at the bottom of the character selection screen.
Successfully complete the game as Ryo under the very hard difficulty setting in single mode to unlock Yamazaki.