I'm thinking that there's two crystals (Light and Dark) and both have warriors in order to maintain the balance.

However I'm pretty convinced that The Twelve are the primal of the wandering races. In fact, Ifrit says so to the adventurers in 1.0 that we can summon our own primals.. which gets me thinking that perhaps our primals could also include light/dark primals - e.g Zodiark?