Akuma - After unlocking Sakura, Dan, Cammy, Fei Long, Gen and Rose, play with someone you've beat the game with and get at least one Perfect and use no continues
Cammy - End the arcade mode with Crimson Viper
Dan - End the arcade mode with Sakura
Fei Long - End the arcade mode with Abel
Gen - End the arcade mode with Chun-Li
Gouken - Complete the game with Akuma, then get at least 1 perfect, 3 Ultra Finishes, 5 first attacks and don't use any continues
Rose - End the arcade mode with M. Bison
Sakura - End the arcade mode with Ryu
Seth - End the game with all the other characters