After re reading tPoD i discovered who i think Mesaana is..i believe she is the sitter Shevan of the brown ajah...In Chapter 25 we notice that Shevan is wearing a dress of "dark golden silk" that from Elaida's POV..then later in that chapter it is Alvi's POV and after Mesaana is done giving her orders Alvi bends down to kiss Mesaana's dress..and she notices that she is wearing a dress of "bronze" silk...in those orders that Mesaana gives Alvi...she tells her to find out why the heads of the ajah are meeting in private..now since she is sitter a lot of people would say that if she was a sitter why would she be asking that...only Ferane Neheran and Suana Dragand are the only Sitters who are actually heads of their Ajahs....so since Dark gold and bronze are prerry much the same color..and Mesaana would want to know what her "fellow" sitters are up to without her being able to know...then that would prove Shevan is Mesaana.