Akuma isn't one of the original Street Fighters. He made his first debut in Super Street Fighter II Turbo, the fourth version of Street Fighter II. Though, you might not have seen him unless you were an especially proficient player or had access to cheats. Akuma began as a secret character. Rather than only remaining hidden for one game, he's remained a secret through many installments in the series. If you wanted Akuma, you almost always had to work for him.