After unlocking all characters except for Seth, Gouken, and Akuma, play Arcade mode with Ryu. During the first two fights, win them with nothing but the "Jump in kick and sweep" method (Fierce Kicks only) and get a "Perfect". For the third and fourth fights, repeat the method but instead of finishing them off, take a beating to fill your Ultra combo and unleash it to finish them. For the remaining fights, repeat the "Jump in kick and sweep" method to finish the fights (no Ultras necessary). If done correctly, Akuma should appear after defeating Seth. Defeat Akuma to unlock him.