First of all, some very amateur analysis:
What do we think we know
-Not as strong in all stats as a Servant should be, could be a more modern Servant, one not known for fighting or not a Servant at all.
-Good at taunting and poetry, uses illusions and general mindfuckery.
-Made us "choose" to hesitate and play defensively, even though he admitted that it benefited him. Looks like his fuckery takes time to spread.
-Acceptable fencer for a Servant, but nothing special.
-A fae of some kind, with at least weakness to iron.
Speculation
All the "not a Servant, not detectable by Mads" vibes, this Servant may be a fusion of a dramaturge and their character.
It may also be a dramaturge-fencer, of which they are quite a bunch. I would say Cyrano if I couldn't see his nose.
I could see pretty much all writers or dramaturges doing the conceptual pov bullshit that he's doing, so that's not very helpful.
All in all, the most plausible candidate I can think of right now is Shakespeare. Associated with faeries (Midsummer's dream), English, and if you're a British mage and are going to summon a poet, you are going for the Bard.
If anyone can think of a dramaturge associated with Faeries and/or being a fencer, please say so. Calderon de la Barca? Don Juan Tenorio? I don't know much about english or french theatre.
***
Finally, about the vote. Not voting, even if it works, looks like is following the guys initiative. If we are going to do something weird like this, I would vote for rejecting his manipulations.
[X] Keep pushing. Whatever 'Fencer' has been doing in his fight, he said himself he needs time. Keep pushing as hard as possible before the situation gets worse, and he'll crumble.
[X] One of you suffers a serious injury.
-[X] Mademoiselle.
Edit: Changed this from Command Seal as I didn't remember we had medical facilities and Alchemist!Cedric as 7h Hex said.
Who knows, maybe it will work. Maybe we can actually vote, and it's our perception that's being manipulated. In the worst case, nothing happens and is the last, default option that happens. I would recommend everyone to make a choice in the first just in case.