I'm on really shaky ground here since it's been like five years since I read the novels, but I think part of the deal Celia cut with the aliens was that her brother and sister--Beckenham and Futaba, who were killed in the war--would be reborn again and again, indefinitely. Well, more like an endless series of their clones would be produced in Dustvin. Which is probably why the Guardian gene expresses most strongly in Shannon and Raquel (their most recent incarnations): Pacifica was created using Celia's own DNA.