She is the original Sortiara. Near the end of the Magusar story arc this is further explained.
Weak to venom.
Spoiler warning! This section contains plot details about the game.
Via the use of the Chalice she created a copy of herself to fill the void of loneliness left behind when Magusar takes possession of the Nameless Sorcerer's body after a long time. You can notice that Magusar reacts more cold the further you are in the story. In the begining he seems very open en friendly. Yet the more you progress the story the more Magusar takes over the Nameless Sorcerer.