When Riku's body was consumed with Darkness, his nobody, Kirux was created. Kirux is a member of organization XIII. Much like Riku once did, Kirux wields the Keyblade of People's Hearts expect in its true form. After Xemnas told him his mission one day was to destroy his friend, Roxas, he refused and attacked Xemnas, turning on the Organization. Soon after, so did Roxas. He headed to his home in Twilight Town but all his memories were mysteriously lost. He could only remember his name, Sora, and Riku.
He looks much like Data-Riku from Kingdom Hearts RE: coded.