Nearly everything about this team is muddled in deception. In the first part of the story, we are told that Seimei was murdered, and that Soubi was his fighter. After Soubi teamed up with Ritsuka, he received opposition from every side, and his Beloved neck marking bled whenever he "betrayed" the name. This change of allegiance is something he fights for throughout the entire story.
In the second part, Seimei's death is slowly revealed to be a ruse. Seimei knew Soubi was a "blank fighter" (someone who is not tied to any sacrifice from birth), and that Ritsu specifically chose Soubi to be Seimei's when he was just a child of fourteen. While telling young Soubi the news, Ritsu mentioned that Seimei might have another fighter out there, but it's unlikely. Given that Nisei sports a Beloved marking on his hand in volume 7, there is the very likely possibility that Nisei may be that fighter.