I'll reiterate, stories don't matter much in fighting games! But, I will tell you about the new character back-stories nonetheless. Ryu has ultimately been corrupted by Akuma with Dark Hadou, the power that both Ryu and Ken are honed in, thus forming Evil Ryu; who wants to quench his new thirst for fighting to the death. Akuma has removed his prayer bead, a necklace that kept him from going insane with power and allowed his true power to show, thus Oni Akuma has been born; truly the evil incarnate. The twins Yun and Yang have come to fight in the tournament, because Chun-Li is fighting in this tournament and seeing as that's the case, they are naturally interested and want to test their own fighting strength.