Destiny 2 is an MMO Video Game. In a similar vein to Final Fantasy XIV, everyone who plays through the game's story is the main character of the story. Both have customizable characters, so the Guardian/Warrior can look however the player wants, be whatever race the player wants, and be whatever class the player wants. Simply put, The Guardian is whichever one of your three characters you are playing as. Even in lore, we are just referred to as The Guardian and are referred to as "they" in this particular lore tab: https://www.ishtar-collective.net/entries/a-deals-a-deal That way, we can slot in the Guardians we create to be our characters into that spot in the lore. Given the Raids are canon, we can assume that the Guardian typically works with a fireteam of three, or six Guardians, with the other five being of a similar level of skill as the Guardian.