The Identity of Anath

It’s plain and simple. No long paragraphs needed. Just the facts.

Description of Anath in Winter's Heart

Chapter 14, "What a Veil Hides", PoV = Tuon
1) It was less than two years since Neferi's [her former Truthspeaker] unexpected death
2) she still was not comfortable with her replacement [Anath]
3) Anath was standing [...] in unrelieved black silk
4) outwardly undisturbed by the chill wind in spite of her lack of a cloak or cape
5) A slender woman, she would have been tall for a man
6) Her charcoal face was beautiful [...] large black eyes
7) named by the Empress [...] when Neferi died. A surprise, with Neferi's Left Hand trained and ready to replace her
8) You certainly were not supposed to be afraid of your Soe'feia, yet Tuon was
9) she cursed Neferi for dying by something as stupid as falling down stairs
10) Anath drew herself up, lips compressing. Her eyes glittered. "I am sure Suroth only had the best intrests of the Empire for ambition."
11) Anath purred. "you must not let him [Rand] learn how dangerous you are to him until it is too late for him to escape"

Chapter 17 "Pink Ribbons"
12) The too-familiar Seanchan drawl seemed absent entirely.
13) "Suroth has the situation here well in control"

Description of Semirhage in Lord of Chaos

Prologue: The First Message
14) Not even Demandred suggested to Semirhage's face that she wore black so often because Lanfear wore only white
15) Semirhage took delight in cruelty
16) Semirhage raised her eyes... unblinking dark eyes in a smooth dark face
17) Semirhage stood taller than most men

Chapter 6, "Threads Woven in Shadow"
18) She did not like having to hurry. And to be called away from her charge; the girl was willful and obdurate, the circumstances difficult.
19) "Your owner does not take good care of you" she said [to Cabriana's warder]

Physical Description

a) 3 agrees with 14 (always wearing black)
b) 5 agrees with 17 (tall even for a man)
c) 6 agrees with 16 (dark eyes, skin)
d) 12 suggests Anath is not seanchan by birth
e) 4 is highly suggestive of concentration used to repress heat or cold, which Aes Sedai, Ashaman and Forsaken are familiar with

The Death of Neferi

* From 1 it was "unexpected"
* Have a look at 9. How do you die from falling down stairs? Must have been a long staircase. Or else...
* From 7, why did Anath replace Neferi when his left hand was trained and ready? Sounds strange to me.
* From 1, Neferi died 2 years ago. Hm... wouldn't that put it at a little after the forsaken were released? Oh. Well.

More about Anath

-) 2 and 8 further add to the unnaturalness of Anath, and also agree with 15 - Anath seems to love this job
-) 10 and 13 shows that Anath supports Suroth, who is high among Darkfriends
-) 11 implies Anath is anxious to see Rand go down / be controlled

Hints from Semirhage's PoV

k) 18 is almost definitely a reference to Tuon. She is young and she is female, matching the description. Furtheremore Tuon herself admits she is headstrong in book 9, which agrees with "willful and obdurate".
l) Consider 19. Why did she call the Aes Sedai her warder's owner? Sounds a bit unusual coming from a mainland perspective. But if you've been living in Seanchan where slavery is common, I guess it would be understandable...

That’s it. Can’t make it more simple.