As has been mentioned before, the ascians appear to be the warriors of darkness. It may one day turn out that they're the "good" guys... but I doubt it.

I find it odd that so many people mention Gaius, but nobody has mentioned Nael Van Darnus. As the main villain of the Seventh Umbral Era storyline of 1.0, he embodied light. White armor, bright laser attacks, and seeking to purify a heretical land that put their belief in false gods, Nael was a holy, "light" bad guy, much in the way Alexander was in FFXI (and one could also draw parallels with Gaius and FFXI Odin)