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Unless it was misdirection, there was another big hint that William is the MiB during this episode: When the MiB and Teddy discover the bodies supposedly slaughtered by Wyatt, the one surviving host (the female one who betrays and stabs Teddy after he rescues her) was the same host that welcomed William to Westworld. We saw this earlier in the season; she guided him to the room where he picked out his cowboy outfit and he asked her if she was real. When MiB encountered her in last night’s episode, he recognized her and remarked that he thought she’d been decommissioned, adding that Ford never wastes a pretty face. This squares with the idea that he had met her long ago, in a different context, exactly as William did.