He is the patron deity of the Shinobi, and is often sworn by among members of the race. It is said he created the Shinobi, only to lament over the fate of his creation when individual members turned violent against one another. He felt every drop of blood shed like a sword stabbing his heart. He might have become a demon, or wiped out his creation under those circumstances, but instead his goodness and love prevailed and he was offered a third option by his lover, a goddess who sacrificing herself so he could be reborn as a mortal.
Orpeus was reborn and used his music to bring peace, camaraderie and honor to the Shinobi, but eventually abandoned his position when he realized his love had taken over the torment of sin and died in his place. He sung a song beautiful enough he could guide himself to the afterlife and went there to retrieve her, only at last minute to turn around and fail in his quest.
In the last years of his mortal life, Orpheus wrote tragic prophecies of the ultimate fate of the Shinobi world, and one of his own reincarnation and rebirth in the penultimate chapter of their ill fated planet. Nero explains all of this story to validate his claim to godhood, as he is the reincarnation of Orpheus.