Saya is the former Imperator of the Novus Orbis Librarium, and the younger sister of Jin Kisaragi and Ragna the Bloodedge. She is currently being used as a vessel for the Goddess of Death, Izanami who is the True Main Antagonist of the Series.
Under the possession of Izanami, it is clear that Saya has become devoid of all emotion and extremely literal-minded. She is obsessed with making the world die and will stop at nothing to accomplish her goal. Cutscenes shown in Ragna's story show that Saya was very dependent on her brother when she was little, and was a normal and confident young girl. Even as she is being possessed by Izanami, the Goddess remarks that she can 'feel' the strong attachment that the real Saya has towards Ragna. Izanami is devoid of all emotion and extremely literal-minded. She is obsessed with making the world die and will stop at nothing to accomplish her goal.
Saya was born with a frail body, and throughout her life, she was bullied by her brother Jin. Her other brother, Ragna, was more caring, spending much time with her. Jin grew jealous of the relationship Ragna and Saya shared, and eventually attempted to kill her, immediately after she gave Jin the Nox Nyctores, Yukianesa. She was kidnapped by Yūki Terumi the same night, and was given unconscious to Relius Clover. According to the Drama CD The Wheel of Fortune, Relius and Terumi were planning to use her body as a "Vessel" for something, later revealed to be the Goddess Izanami.
Since the day of her kidnapping, her appearance has changed vastly. She now has red eyes and long, purple hair. It is also interesting to note that she does not seem to have aged at all. Also, her personality is completely different, like she is not acting like herself, and she speaks in an archaic tone. As Imperator, she rules the world and goes by the name "Imperator Librarius".Izanami is an immortal being basically Death incarnate and wants to create a world of death by using the BlazBlue Embryo Turing all life into Seither and making the true BlazBlue. She also has a feud with Rachel and used Hazama and Relius as pawns and abandons them to their fate after they failed but took Phantom with her as well as a Brainwash Ragna.