Evil Ryu and Oni Akuma in Super Street Fighter IV Arcade Edition is available for quiet some time now. Both of them are hidden unlockable characters.
He was originally introduced in a Street Fighter Zero manga series authored by Masahiko Nakahira and later adapted in the Street Fighter canon by Capcom. He is the version of Ryu should he succumb to Satsui no Hadou, and depicts Ryu’s struggle to resist the urge. Gameplay-wise, Evil Ryu has more powerful attacks than the original, strikes faster and possesses some of the attacks of Akuma (such as the Shun Goku Satsu). Like Akuma, Evil Ryu has a signature symbol that appears when he performs the Shun Goku Satsu.