"Our agents are everywhere, appearing as you do, and there is no way for you to know who is true and who is false
-Omen
Well after seeing this quote it got my mind rolling on the possible Undines in Cythera (Excluding Pelagon, Omen, and Tavara) I have managed to actually zone it down to a few people who are actually Undine. Atreus, the gem trader down in Kosha is the most obvious one, if you examine Tavara's diamond ring it will say 742 - Will to Union, then if you wait for Atreus to fall asleep and look in his desk he will have a plain gold ring saying 742 - Will to Union.
I had also remember reading on the boards that Nothing, the guy in the upper lef hand corner of every map, was a possible Undine, however there is no physical evidence of this. So far I think he is just that, Nothing, a template or something but otherwise of no importance.
Joppa, or as I remember him Timon's sister's crazy assistant. They've been exploring the ruins unharmed for who knows when and then when you arrive Larisa is packing up and heading back to Kosha. My personal theory is that Joppa killed Larisa and the guard and then took the guard's form(why else would he stay there). Why you ask? To make sure you took the Crolna, notice how Omen didn't speak to you when you took the crystal.
The three brother's of the House of Comana, may or may not be Undine, but at least they are aware of them. When you are completeing find Opheltius's Murder Weapon, Darius (one of the three brothers) both accuses Halos and defends Pelagon.
Any other thoughts?