This question also goes to the traveler are both the witness and traveler emissaries of the winnower and the gardener.
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Neither. The light and darkness are neutral (but counter forces) as such they don't need emissaries. The Witness is just a being that stumbled across the darkness and learned how to use it's power. I suspect he was a creature in the black garden and Witness the light and dark and learned how to use the darkness powers once the light left the garden.