Kujaku first appears as a mysterious traveler that Ashura finds near the Yasha village after he falls out of a tree (supposedly from hunger). He comes and go at random, but he almost always assists Yasha and Ashura's group usually in the form of information, magical aid, or light babysitting. His powers range from the highly destructive to the prophetic. But as his motives are unclear most of the Six Stars members do not trust him. Yasha endures his presence mostly because Ashura likes him a great deal; a sentiment Kujaku genuinely reciprocates. But because of his odd traits and purple eyes, he is accused of being a mazoku (a demon).
In actuality, Kujaku is the son of the former emperor and his sister Sonsei-ou, who was also the head stargazer/priestess before Kuyou. This would imply that he and Kisshou-ten are half-siblings. As a child he manifested a daten (a third eye placed vertically in the center of the forehead) and a pair of black feathered wings as marks of the fact his parents committed the sin of incest.
It is revealed near the end of the final battle that Kujaku has used his powers to attempt to change fate: He forged a sword which is connected to Shura-to and so allows a measure of control over Ashura, Yama-to, which he gave to the Yasha clan so it would pass down to Yasha-ou. He explains that initially he did it out of curiosity, to see if fate could be changed; and then later was motivated by his affection for Ashura as they were both "unneeded children".
Kujaku dies in the final pages of the manga after sacrificing himself to free Ashura from his sleep.