From Tokyopop:
A lonely yet caring 15-year-old Chiaki, the half-angel daughter of the Archangel Michael and a human woman, welcomes into her home a dejected devil Ororon who pledges to stay at her side forever.
Both of them have been ostracized from their respective celestial realms--Chiaki because her origins are impure and Ororon because he is the youngest son of the Lord Devil and therefore a threat to his older brothers for leadership of the underworld.
As various agents of heaven come by to destroy the angel Chiaki, she learns that having the incredibly powerful Ororon by her side is useful. And as assassin after assassin comes to eliminate Ororon, he learns that Chiaki can teach him a valuable lesson about the sanctity of life.