Do not read further if you don't want spoilers for Nightfall. Some of the comments here also contain spoilers for earlier books. Further down, I have a comment with spoilers for Flashback as well.
So, in book six, we learn that Mr. Forkle can no longer be in two places at once. This means that if he (or one of his known identities) is standing in the same room with another character... he can't be that character.
This may not actually be worth much... but it will help us start narrowing down the list of people that he might be.
So, the following is a list of people that Mr. Forkle has appeared with during book six. Some of these will be very obvious (like, duh, he isn't Sophie; he's probably not a goblin, either), but I'm just trying to cover all the bases here.
Mr. Forkle CANNOT be:
Lady Gisela (didn't appear together physically, but she was hailed while Mr. Forkle was in the next room over from Sophie and Keefe)
Depends... it could be, I guess... but we're saying that the author probably won't make any alter egos different genders because it's difficult physically and politically, and I said that the political part was a shame.
Hummmm.... It is also possible he was delusional. But, for the sake of simplicity, I'm going to assume he had about five. We can rule out other Foxfire teachers, since there was only two of them, and so meetings would be weird.
Ha, that would be horrible if he was just delusional. "Psyche! They actually were ten different people. Surprise!"
We can be pretty certain he's not a councillor given the way he reacted to almost being elected as Leto, but I don't think there's enough evidence to rule out other teachers. I agree it would be a little odd, but I think it's still possible.
I don't know if this has been said but it also hasn't been mentioned which life was the life that Mr. Forkle originally grew up in. Are we counting Mr. Forkle and non ruckleberries Mr. Forkle as different people?
Do not read further if you don't want spoilers for Flashback.
Again, just listing every single possibility here, regardless of feasibility. In Flashback, based on Mr. Forkle's interactions, we now also know that he cannot be...
(List of every new person we can add - that is, characters who aren't on the list from Nightfall. This list is... disappointingly small, if I'm being honest here.)
Cadfael (plus his assorted unnamed ogres)
Lady Gisela (more or less confirmed now)
(Full list of everyone he appeared with in Flashback (for curiosity's sake?))
Lady Sanja (we don't see her, but he claims to have talked to her)
Lord Cassius (ditto)
The rest of the Collective (Granite, Wraith, Blur, Squall)
Any current Councillor (Alina, Bronte, Clarette, Darek, Emery, Liora, Noland, Oralie, Ramira, Terik, Velina, Zarina)
There are also a bunch of Neverseen members we can rule out, because presumably Mr. Forkle was at the festival while they were at Everglen/the tower... not that I think he'd be a member of the Neverseen. But still.
I've read Flashback once already, and I'm reading through it more carefully a second time. For compiling this list, though, all I really need to do is scan each page for Mr. Forkle's name(s) and pay special attention to any other names that are mentioned in scenes where he's present. And since about half the book takes place in Foxfire, I've mostly only needed to look for one name (Magnate Leto) so far.
Once I get back up to page 506 (where I left off for the list), I may just write names down as I encounter them.
Well, maybe (*new character in book 7 mention*) Tinker built him something so that he can make it seem like someone else is the room, but only when he's there? Sounds like the kind of thing Mrs. Messenger would do. xD
wasn't one of his identities the doctor that implanted Sophie in her birth mom?
So that would make these his known identities:
The fertility doctor
(Not saying this is definite, just writing my thoughts)