A long time ago, Zeus, the great god, created a three-tier hierarchy of angels, top, middle and lower, although they were supposed to be equal. And Zeus also appointed 6 angels as Saint Beasts who govern the animals of Earth. They were: Gou, the seiryuu; Shin, the genbu; Rei, the suzaku; Gai, the byakko; Judas, the kirin; and Luka, the houou. However, Kira, the shooting star, and Maya, the fuuga, did not attend the selection tests and left for the Earth.

The six angels fulfilled their tasks faithfully. However, because of Zeus' increasingly tyrannical acts, they begin to mistrust him.

Source: AnimeNfo