The answer is that every individual player plays the canonical guardian. You can see it as everyone plays the events of the game on their own timeline that only differs in a single way (who "The Guardian" is) For every players game every other player is simply another guardian but the player is "The" guardian. So if youre an Exo Warlock then The Guardian is an Exo Warlock, if youre an Awoken Hunter then The Guardian is an Awoken Hunter. Events the player has not done are not done by The Guardian but by other Guardians of that timeline, once the player has done them then you can technically argue you shift timelines to one where your Guardian is The Guardian who did those first.