Possible spicy opinion but I actually think Hythlodaeus is us, we were Hythlodaeus. Literally no one else can see Ardbert, except us, until Hythlodaeus confirms they can see him too, albeit faintly, commenting on how recognizable the hue of our soul is. Emet-Selch acts astonished to see an Ascian before him when we rejoin with Ardbert, briefly seeing us as our fully "restored" self.