My only reluctance with saying it's Hydaelyn (although I do
think it resembles her anyway, due to the form the High Seraph bears for her upper body), is that the insect part is a better fit with the lower body of the High Seraph. Still, there's no context to go with it, so I am eager to see when and why it's going to pop up. As for the number of Ascians, because the Esper names and sigils are purely a hat tip to XII, it allows them to keep the 14th's identity secret for now. I am sure the Ascians have elevated someone to this office in the meantime, possibly even a fragment of the WoL's Amaurotine self, although that may come down to how tempering works (i.e. did the WoL's ancient soul summon Hydaelyn, and if so, were they tempered, and would this endure beyond the point of being sundered?) With only 13 Espers but 14 Convocation members, they have free reign to keep the gender, sigil and office a bit vague for now, and map some attributes to your character.