Aoyagi Ritsuka’s brother, Seimei, died early because of an accident, thus giving Ritsuka amnesia.
Ritsuka meets Agatsuma Soubi on his first day of school. Soubi claims to be his brother’s “fighter” and because of his death, he was now Ritsuka’s. The world of master and servant and spell fighting is later revealed to Ritsuka as Seimei’s world.
I won’t divulge further about Loveless since it’s a shonen ai anime. I hear the manga is yaoi too. I prefer to get out of the Sanriotown Moderators’ way. 8)