Loveless

Twelve-year-old Aoyagi Ritsuka is left with his insane mother as his only family when his brother, Seimei, is killed suddenly. After moving to a new school, he meets Agatsuma Soubi, who claims to have known his brother. Ritsuka eventually discovers that Soubi and Seimei used to be a fighting pair, whereby Soubi was the "Fighter" and Seimei was the "Sacrifice." Now that Seimei is gone, Ritsuka has inherited Soubi, who will become his "Fighter. (myanimelist.net)