You were Muichiro Tokito's best friend... Now you are Upper Rank Zero... Muichiro doesn't remember about you because of Severe trauma... And you don't remember him either... Since your a demon you forget everything about your past.. But you were a demon slayer... One day you learned that Muichiro Tokito, turned into the new mist hashira, again, you don't remember him and he does not remember you since you look different as a demon then as a human... But since Muichiro is a hashira he kills demons and you kill humans... Your blood demon art is," shadow/sun/moon breathing..." So you have to use a Nichirn Blade like Upper Moon One...