Defeat all opponents before reaching M. Bison in less than twenty minutes without using any continues. When you fight M. Bison, Akuma will appear, perform the Shun Goku Satsu on Bison, then fight your character.
In Classic Arcade mode, press jab to select a character. Then, as the airplane flies to the top of the screen press the corresponding sequences of buttons and press jab again to unlock the following Classic characters:
To play as Akuma in Classic mode, highlight these characters in the following order and wait 2 seconds: Ryu, T. Hawk, Guile, Cammy, and Ryu again. Press Start and all three punch buttons in quick succession. Akuma's portrait will replace Ryu's if done correctly.
To play as Akuma, defeat all fighters leading up to M. Bison without continuing in under 20 minutes. When you battle Bison, Akuma will appear. After Akuma appears, highlight Guile on the Arcade Mode's character select screen and press "Up" to select Akuma. To select Akuma in Vs. Mode you must choose E. Honda and press "Up" to select Akuma. To select Akuma in Classic Mode or Training Mode Classic, highlight Ryu and simultaneously press LP+MP+HP+Start.