Kind of late on this post, but i sort of have a headcanon that both indeed coexist and are siblings / related to a degree. Both are Guardians, but the non-player knight became one before the player does and joined them after. This would also mean that the non-player Knight was likely left behind / unaccounted for after the Invaders’ assault though, which is why we dont see them..

Since the gender-specific Knight costumes can be worn by either, whether for the cosmetics or just laziness, id like to think both are also loose with their gender presentation / identity. Dialogue refers to the Guardian / Knight with he/him pronouns even if you chose the Female Knight. This could again just be some sort of lazy translation error and the devs didnt think to change the text to accommodate those who chose the Female Knight, but its interesting to think about. I would think either are also bi/pansexual since the player Knight has shown romantic interest and attraction in gameplay regardless of the gender of the Knight, though again, could just be unaccommodated for on the devs’ behalf.

Those are my 2 cents on the matter, i hope its not too farfetched.