The Celestis made him from the germ-plasma of their slave class as part of an experiment. He was raised in the doom-creches of Mictlan's Baby Farm, surrounded by ornamental skulls and faceless wet nurses.
After serving the Celestis for a considerable time, One was influenced by the hermit into going rogue. While sent to clean up the Celesti operation in the Seventh Retro War, One intentionally went against his orders and let some pockets of resistance survive. Because of this, Two was assigned to observe One.
The Celestis sent One and Two to find out what had caused a Lord Celestial to not only cease to exist, but to have never existed.
In fact, One was responsible for the non-existence of the Lord Celestial; he was working with the hermit to destroy Mictlan before it would attract the attention of the Swimmers and cause the universe to be destroyed. To that end they arranged for the Fendahl Predator to be released from its time loop prison. Since it fed on concept, and Mictlan was a conceptual construct, the Predator could not resist feeding on it. The inhabitant whose existence it first erased was the one that caused One and Two to be sent to investigate in the first place.