[Tony Zbaraschuck, Marc Zappala, Pam Korda, Leigh Butler, Jennifer Liang]
In LOC, we find out that Mesaana is hiding out in the White Tower [LOC: 6, Threads Woven of Shadow, 138]. Naturally, this leads to the question of who she is pretending to be.
RJ has had a couple of things to say on the matter. He told "Tallis" at the Harvard Coop signing [January 18, 2003] that "there are many clues as to Mesaana's identity, enough that we should figure it out before COT. He basically said that he'd full-out reveal her in upcoming books, though: '...and if you still don't know, well, you'll find out later.'"
Yeah, but we want to know now, don't we? When asked if we have actually seen Mesaana's alter ego in the Tower, RJ said, yes, we have [Robert Mee, Bailey's Crossroads signing, VA, January 23, 2003].
Of course, that doesn't narrow it down all that much, but it does at least mean that Mesaana is not likely to be some random servant in the Tower, or a sister we haven't been specifically introduced to.
There is some difficulty with the idea that Mesaana is posing as an already existing person that people know. None of the Forsaken (that we know of) have copied an already-prominent identity. Rather, they take a little-known or unknown identity and then promote themselves rapidly. Supplanting a well-known personality is not easy; it requires a total control of oneself, and the ability to copy the tiniest habits. This is especially true in the Tower, where Aes Sedai can doubtless read meaning in the tilt of an eyebrow or the tapping of a finger.
However, in [ACOS: Prologue, Lightnings, 41], Alviarin thinks "She [Mesaana] must be one of the sisters; surely she was not among servants, bound to labor and sweat. But who? Too many women had been out of the Tower for years before Elaida's summons, too many had no close friends, or none at all." Thus, Mesaana could be posing as one of the long-gone sisters, somebody who'd been gone so long that differences in personality, etc. would not seem too amiss, or one of the AS who is not known very well. (This is discussed further below.)
In [ACOS: Prologue, Lightnings, 41], Alviarin notes that Mesaana taps her lip with her finger while thinking. Unfortunately, numerous Aes Sedai do this, among them Leane, Alviarin herself, Nesune [LOC: 27, Gifts, 395], Theodrin [ACOS: 11, An Oath, 210], Moiraine, and Vandene. So the lip-tapping thing is a fairly useless clue, and probably a red herring to boot.
When Mesaana's disguise is shattered by Shaidar Haran in [COT: 21, A Mark, 516], Alviarin sees "a blue-eyed woman... a tantalizingly familiar woman who looked just short of her middle years." This strongly suggests that Mesaana is pretending to be a sister that Alviarin knows.
Why? First, the fact that Alviarin finds Mesaana's face familiar at all means that it's part of her disguise (i.e. she's not walking around with a completely different face on). Secondly, note that Alviarin puts a definite age range to the Forsaken. Put these two facts together and then ask what factor could make someone's face somewhat familiar but not completely? Adding or removing the Ageless look, that's what.
Mesaana's real face is not Ageless, but she would have to add Agelessness in order to pretend to be a sister. As we know from Siuan and Leane's escapades (and other examples as well), losing the Ageless look changes your appearance sufficiently to make it difficult - but not impossible - for others to recognize you. So it appears that rather than go the whole hog, Mesaana has been opting to keep her real face and just make it Ageless. There's no other reasonable explanation as to why Alviarin would only kind of recognize Mesaana's face.
There have been objections to this idea, mainly along the grounds that keeping anything of her real face in her disguise seems like a rather large risk to take. As long as you're going to go through the trouble of making your face Ageless, why not change it completely while you're at it? [Jim Mansfield]
Well, for several reasons. One, it's valid to assume that the more elaborate the Illusion, the more difficult it is to create and maintain over long periods of time. The first lesson in successful undercover work is to keep things as simple as possible. Two, with the reasonable supposition that no other Forsaken besides herself are going to be wandering the Tower, why would Mesaana bother? Who besides the FS would recognize her true face? Much easier and less bother and effort to Ageless-ize her real face than to make up another one. Third, there's precedent for it: Lanfear's "Selene" disguise was basically just a younger version of herself, and Egwene notes that Moggy's "Marigan" face is basically the same as her real face, except with careworn touches [LOC: 37, When Battle Begins, 489].
When Alviarin is groveling before Mesaana in [TPOD: 25, An Unwelcome Return, 497], she catches a glimpse of Mesaana's skirt: "Seizing the hem of Mesaana's dress, she rained kisses on it. The weave of Illusion...did not hold completely, with her frantically shifting the skirt's edge. Flickers of bronze silk with a thin border of intricately embroidered black scrollwork showed through."
The finery of Mesaana's dress cast further doubt on the possibility that she could be masquerading as a scullery maid or some such, though it's been pointed out that Mesaana could easily have deliberately changed into the silk in order to throw Alviarin off the scent or to test her loyalty [Dennis Higbee]. However, in COT she is wearing another silk dress, this time green "embroidered with elaborate bands of bronze." This second dress is more significant a clue than the first, as Jean Dufresne points out, because Alviarin had just pressed the panic button to summon Mesaana, which means that whatever Mesaana had to drop doing at a moment's notice, she was doing it in the silk dress. This again reinforces the idea that Mesaana is masquerading as someone who wears silk regularly, i.e. a sister.
The bronze color of the first dress led us to look among the Yellow or Brown Ajah for Mesaana, since Aes Sedai tend to dress in their Ajah colors (though that is only a tendency, not an absolute - Alanna, for instance, has been seen wearing blue and yellow, and she's a Green). The second dress, while green, also has bronze in it, and Rich Boyé points out that Mesaana is described as wearing "russet" in her inaugural appearance in the series [LOC: Prologue, The First Message, 55]. It's clear from Demandred's comments at that meeting that Mesaana was already ensconced in the Tower at that point, so it remains uncertain whether Mesaana's tendency to wear brownish colors is part of her disguise or just a personal predilection. In any case, it still factors in our consideration of who she might be impersonating.
The last thing we should consider before moving on to specific candidates is Mesaana's strength in the Power. As a Forsaken, obviously her strength would far outstrip anyone else's in the Tower, and so must be disguised. A lot of people have used this as an argument against her pretending to be a sister, because we have had no direct evidence that you can only partially disguise your strength in the Power. Most examples of FS hiding their strength in the OP (Moggy as Gyldin/Marigan, Semirhage as Anath) have involved masking the ability completely, and indeed Mesaana does completely hide her ability when talking to Alviarin; but if Mesaana is pretending to be a sister, she clearly can't have it appear that she can't channel at all while walking around being her alter ego. Furthermore, Alviarin mentions that the others of the Chosen she had met let her sense their strength, how far above her they stood, and Mesaana was the only one who hid her ability completely [ACOS: Prologue, Lightnings, 41]. If Mesaana could partially hide her strength, why would she feel the need to hide her full strength when behind the Illusion? [Rajiv Mote]
Therese Wikström counters the latter point: "Sisters establish a pecking order based on strength in the Power. Hence, they know pretty well how strong any other Aes Sedai is. If Mesaana is posing as an Aes Sedai, and masking her ability to channel to a degree (say, from a 15 to a 8 on a scale from 1 to 20), she wouldn't reveal her secret identity's strength to Alviarin. If she did, Alviarin would only have to search among sisters of a certain strength."
That still does not explain, however, why Mesaana couldn't simply have chosen to appear to Alviarin showing her full strength. One possible explanation for this is that it is a side effect of reversing weaves (see section 2.3.18). Alviarin never senses any of Mesaana's channeling at all, which indicates she is reversing the weaves; if reversing hides the glow of someone embracing saidar, maybe it also hides the fact that the person can channel at all.
John Nowacki reports that RJ said, at a post-TPOD booksigning in Washington, DC, that a channeler can hide strength as well as ability to channel, but added that few people know how to do it and the AS don't even know these tricks are possible. There is also some indirect evidence of it from Lanfear, who disguised herself as Else Grinwell to move about the Tower [TDR: 24, Scouting and Discoveries, 224-225]. Else was a novice, so logically Lanfear couldn't have simply made it appear that she had no channeling ability, but then she certainly couldn't have her walking around radiating "Strongest Female Channeler Ever!" either.
Basically, with regard to the strength question, the evidence that Mesaana could partially hide her strength is not ironclad, but combined with RJ's say-so it'll have to do.
The two main candidates for Mesaana's secret identity are Tarna Feir and Danelle.
One objection to both characters we should address at the outset is the problem of prior histories. It's pretty clear that both characters existed as "real" people in the past - Tarna for certain. There is her association with Galina, for one thing, and Pevara appears to know Tarna fairly well, commenting on her attitude toward men and when she had gained the shawl [COT: 22, One Answer, 521]. Pevara also comments that Elaida trusts Tarna, which indicates that Elaida knows her as well. As for Danelle, her insertion in Elaida's junta implies that she was a known commodity to at least a few of the conspirators. Also, Siuan knew who Danelle was, since she was monitoring her progress in the library reconstruction project. These are commonly raised as arguments against either character being Mesaana, Alviarin's thoughts notwithstanding.
However, it's been pointed out that in such a small and tight-knit community as the Tower, creating a fictitious sister out of whole cloth would have been difficult if not impossible to accomplish. It might be easier to simply torture an existing Aes Sedai into giving up all her personal details, much as Semirhage did with Cabriana Mecandes on behalf of Aran’gar. As long as we assume she's impersonating an AS, and RJ certainly seems to be steering us in that direction, then Mesaana almost had to choose to pretend to be an already existing sister.
But how does that square with our assumption that Mesaana must be using her own face as part of her disguise, as discussed above? The obvious answer to this is that either the sister in question had been gone so long from the Tower that no one remembered what she looked like, or (more likely) Mesaana happens to bear a significant resemblance to whomever she's pretending to be (in fact, that very well may have been how she decided who to replace in the first place).
Tarna is the Red emissary to Salidar who interrogates/lectures Elayne and Nynaeve [LOC: 13, Under the Dust, 232] before disappearing for three books and finally resurfacing in Dorlan outside Tar Valon, citing traffic problems and weather to explain why it took her almost four months to get from Salidar to Tar Valon [COT: 22, One Answer, 524]. In [COT: 22, One Answer, 522] we learn that she has been named Elaida's Keeper in Alviarin's place. However, she receives a POV chapter in KOD [KOD 25: Attending Elaida] which rules her out as a suspect. Which leaves us with Danelle.
Danelle is the dreamy-eyed Brown who participated in Elaida's coup in TSR. She has since been seen several times drifting aimlessly around the Tower, having been shunted to the periphery once Elaida was firmly in power.
Counterargument: Or it could only indicate a personal preference, as noted above.
Counterargument: This can be explained by supposing either that Mesaana is ballsier than she's letting on to Alviarin, or that Mesaana did not replace Danelle until after the coup.
If we suppose Mesaana is Danelle, we see a character who played an essential role in engineering Siuan's downfall but then quickly and quietly faded into the background once it was done. This is a risky move for one of the Forsaken, but perhaps a necessary one to ensure the schism between the Aes Sedai happened as planned.
Other suspects
In The Gathering Storm, Egwene demands all sisters reswear the Oaths and affirm they are not a Darkfriend. Three sisters are unaccounted for: Nalasia Merhan, Teramina, and Jamilia Norsish. [TGS Epilogue: Bathed in Light] However, as Robert Jordan said we should be able to figure it out by COT, it seems very unlikely that one of these sisters would be the Forsaken, as their first mention in the series occurs in TGS. It’s more likely they were captured by the Seanchan during the raid without witnesses to confirm it and only mentioned as a red herring.
In conclusion, both the Tarna-as-Mesaana theory and the Danelle-as-Mesaana theory have their problems, but overall it seems that Danelle is the more likely of the two.