The Seraphim, who has no other known name, was one of the Heavenly Host, a servant of God. He patrolled the border between Heaven and Hell, and he did his job well. Until he met the demon.
She was beautiful, dark and dangerous. Against his very nature, he went to her, and they fell in love. They mated, knowing that they were doomed by doing so, and their child was the being known as Genesis.
When Genesis was born, God fled Heaven. The other angels put the Seraphim on trial and banished him to Earth, where he was seen falling by a pilot for the US Air Force. The US government notified The Grail, a powerful group of apocalyptic Christians, who took the angel into custody.
The Seraphim was held in chains at Masada, the headquarters for The Grail in France. He gave information to The Grail on God's disappearance and the creation of Genesis. When The Grail wanted truth, they took it to the Seraphim.
Hoping for release from his imprisonment, the Seraphim tried to help Jesse Custer learn how to talk to Genesis - he advised that Custer seek out the old ways of the Native Americans. That way, Custer could learn all he needed to if he wanted to control Genesis.
The Seraphim was killed by God, who named him "Betrayer" before burning him to death in the caverns below Masada.