So I was rewatching Hollow Mind and Hunter says that he was the youngest scout ever to become golden guard, and then he says that he never found out what happened to the last one. Which raises the question, did Hunter know his predecessor? I don't think he did. My theory is that Belos doesn't only make new grimwalkers once the current one has betrayed him. I think he makes new ones with some regularity, just to be prepared (since he had that book on grimwalkers in eclipse lake and he sent hunter to get selkidomus scales when there was no indication that Hunter was going to turn against him soon). But then, for some reason, Hunter's predecessor betrayed Belos before he was ready to replace him, so there was a period when there were no grimwalkers who were ready to lead the emperor's coven. (I think the emperor's coven is supposed to be led by the current golden guard.) That's why Lilith was head of the emperor's coven, but then she also betrayed him, so Belos had to promote Hunter earlier than the other grimwalkers.
I just thought of this so it might not make total sense, but what do yall think?