One theory for the notable resemblance between Pacifica and Celia is that Browning had intentionally used Celia's DNA as the DNA for the Providence Breaker, which would explain why the Peacemakers cannot attack her, as Celia and Pacifica are practically the same person.
5000 years ago, he led 26 Dragoons, with their D-knights against the alien force - and later the Peacemakers - in an effort to defend humanity.