XerØ "The Soldier: Book One": is, more or less, the origin of XerØ. Of course, this is Priest, so it doesn't start at the beginning.
It starts 25 years from now, with a guy who looks like Trent, chases tail like Trent, but isn't Trent. It's his son, who has unwittingly brought a Closer home with him ato meet dad, who's writing the memoirs we've been reading as narration. Trent has had 25 years to absorb clues, of course, and he knows she's there there to close him. Of course, before she does so, she wants to know what he's planning to say, as part of her cover ID, so we get the origin story, starting on page 4.
Of course, this is Priest, so it doesn't start at the beginning.
Trane's already a Closer, 67, and not yet a basketball start or a cyborg. In that respect, we're in the story early, but not at the beginning. It's his first mission as a Closer, and it's not ging according to specs, suprise. Meanwhile, Trent is already working for the CIA as a gadgeteer type (we get a condensed version of how he got there), when's tapped by Øne to come work for the Closer branch as well. At which point he knows Trane's dead...at least as Trane.
The mission goes down, Trane gets messed up by trusting the specs too closely, Trent goes to work rebuilding him into the cyborg we know and love, who rapidly works his way up to 12. The final bond between them is seemingly broken when Trane gets the whiteface prosthesis to wear...and Trent then realizes Trane has given up on being a brother, and is fully devoted to being a Closer.
As the first part closes, Trane has become 6. And he IS a number, not a free man. A prisoner in his body, in his mind, in the system.