All characters in the book agree that Mesaana is an AS (though Egwene correctly states that it's basically impossible to play a known person for a long time).

However, if Mesaana is normally disguised as an AS, why are her true look AND her true voice familiar to Alviarin, after SH disrupts Mesaana's Mirror of Mists in CoT. And why is Mesaana then such afraid that Alviarin could recognize her now and therefore want to kill her, if Mesaana uses the disguise of another person. Did Mesaana really find a secluded AS in the WT, who has a familiar appearance and a familiar voice to her own?!

For these reasons, it still makes the most logical sense to me that Mesaana - like most other Forsaken - simply created a new identity of a personal servant, maid, secretary or clerk in the WT, and isn't disguised as an AS. Maybe she is even Elaida's maid Carlya, who is first mentioned in TGS..