So I've always had this question rooming in my brain. Our guardian is able to use (with TTK) all three subclasses and swap between them at our will. But in most cases, with the firebreak order of Titans or the order of the praxic fire with warlocks, they seemed to only use one subclass. So I wondered if all guardians are like us able to choose between subclasses, and just stuck with one, or are they just revived and either just "born" with that class or only mentored/trained for that one subclass.
I also knew that a hunter (I think shin Malphur? Correct me if I'm wrong) "invented" the golden gun killing dredgon yur, and from then on maybe he thought that to other hunters or just all guardians are born with the potential for all 3 subclasses just waiting to be unlocked.
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It's worth noting that successive guardian generations seem to get progressively stronger on average, and our guardian is the most powerful within our generation. This makes us one of if not the most naturally powerful guardian. To our knowledge most guardians probably have some rudimentary understanding of each of the three elements, but are only able to 'master' one and so they only really focus on pursuing one.