well im not sayin that Riku and Sora arent on good terms but from the new trailer u hear Riku say"So this is the true world, well then let it be engulfed by darkness" that kindaa goes against Sora's code if u know what i mean. so u never know that really could be Riku. maybe him and Sora have different perspectives than each other and are forced to fight. maybe but im positive that that unknown is Riku or if it isnt it has to be the BHK or someone we have never seen before. that unknown runs just lke Riku in Destiny Islands. but hey we just have to wait and see.