Kuruizaki no Hana

Long ago, when Japanese gods descended upon humans, their mixed offspring inherited supernatural powers...and the name "Kuruizaki no Hana." Now, those of the Demon world are rising up to wipe out these offspring - led by the cool and confident assasin, Ushitora. He's worshipped as a deadly professional, but when he falls in love with one such gifted boy, Masato, everything changes. Ushitora betrays his people, instead sacrificing himself to a life on the run, in order to care for Masato and a spirited orphaned girl named Nao.

(from mangaupdates)