Plot taken from my feverish mind:

First Ritsuka's brother Seimei, was killed, and then he met someone saying he was a friend of Seimei's. His name is Soubi. Soubi explained that he was Seimei's fighter, and Seimei the sacrifice, meaning Seimei was the master, and he the servant. And that now Soubi belongs Ritsuka's because Seimei had told him to. Seimei and Soubi's true name was BELOVED, while Ritsuka's is LOVELESS.