I would assume she is a Deva:

https://forgottenrealms.fandom.com/wiki/Deva

Being a Deva of Selune, I suspect Shar expected Ketheric to kill her, as a "loyal" Sharran. But instead he got Balthazar to Soul Chain her in order to grant him immortality. I suspect at this point he was already a chosen of Myrkhul, probably due to the death of his daughter (which is ironic given it was the death of his wife, a Selunite, that turned him to Shar in the first place). A lot of the information regarding all of this stuff can be found if you search moonrise towers thoroughly enough.