[SPOILERS] The Convocation of Fourteen

[Wolffriend]

I think there are a few details that imply that Hythlodaeus was the fourteenth. He was once a close friend of Emet-selch, and he mentions that an errant thought of Emet's may be the reason for his awareness. So it seems Hythlodaeus is the only likely candidate we've seen for the fourteenth member, due to his past association with the prominent convocation member, Emet-selch.

But he also refers to us as his "new old friend" which would be a strange way to refer to himself. And he does explicitly say that you are the kind of fate he would have expected for one such as she(he). Again, a strange way to refer to yourself, even your former self.