Akuma is a secret character in Street Fighter 4. Depending on whether the game played is the console or arcade version, there are different ways to unlock him.
Story
Akuma was Gouken's brother, who was the master of Ken and Ryu. Akuma and Gouken chose different paths in their training, with Akuma taking a path that taught him to remove all the compassion he felt for others. This allowed him to master deadly martial arts forms.
Akuma killed his brother Gouken in a fight, and Ken found the body afterwards. Once Ryu found out what happened, he joined Ken in a search for Akuma. Neither has been able to defeat Akuma yet, although each has faced him in battle.Street Fighter Wiki: Akuma
Street Fighter 4 - Akuma Details
Special Moves
Gou Hadouken (Fireball): Down/Right Quarter-circle + Punch
Zankuu Hadouken (Fireball): Down/Right Quarter-circle + Punch (while in the air)
Hyakki Shuu (Demon Flip): ↓ → Down/Right Diagonal + Kick
EX Moves
EX-Gou Hadouken (Fireball): Down/Right Quarter-circle + Punch, Punch
EX-Zankuu Shoryuken (Dragon Punch Uppercut): Down/Right Quarter-circle + Punch, Punch (while in the air)
Super and Ultra Moves
Shun Goku Satsu (Raging Demon): LP, LP → LK, HP
Shun Goku Satsu: LP, LP ← LK, HP
Pros
Very powerful character
Possibly powerful enough to nerf
Super and Ultra moves are very effective
Cons
- Has low stamina
- Lots of power lost when hit
- Can't go blow for blow
- YouTube: Street Fighter IV - Akuma's Ultra Combo (Time: 0:31)