He is a high priest that "speaks" for the Traveler. There have been multiple across history, and Osiris was the apprentice of the current one
English
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We know that Osiris was the Speaker's apprentice, but who do you think Saint-14 is to the two of them? Surely if Saint-14 is connected to the Speaker, wouldn't there be a chance he could also be connected to Osiris?
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Saint-14 recommended Osiris for Vanguard, and went looking for Osiris in the wake of Twilight Gap, at the Speaker's command