The earliest known information about Saya is that she was in a laboratory controlled by Relius Clover with her siblings Ragna and Jin. Her "mother" was the 5th Prime Field Device. She was saved from the lab by Jubei, who took her and her and siblings into the care of Celica A. Mercury, the Sister.
Two weeks after having arrived in the Sister’s care, the children went to find strawberries to make into jam alongside the nun. Here they learned that she had no sense of direction, which lead to them getting lost in dense woodlands. The journey’s length took its toll on Saya, and she retired herself to being carried by Ragna across the forest; as time passed, she took notice of a creature in the background, but was told to ignore it.
After the group found the strawberries, Saya volunteered to help the Sister create the jam while the boys laid the table. What she produced was more akin to blackened sludge than jam, but nonetheless, Ragna ate it – he was knocked unconscious. 
The children would live happily in the church until Yuuki Terumi managed to find Saya close to the edge of the church’s barrier. He handed her the Mucro Algesco: Yukianesa to pass to Jin. On March 1st, 2192, Saya obliged under Terumi’s influence, which lead to the barrier collapsing as a result – the Sister was murdered, Ragna’s right arm was cut off, and Jin and herself were kidnapped by Terumi and Relius. Saya was kept by the duo, becoming the basis for the Boundary Interface Prime Field Devices Lambda-11, Mu-12, and Nu-13 after having been viciously tortured by Hazama in his pursuit to understand pain.  At some point, Saya became the Imperator of the Novus Orbis Librarium, and would also become the vessel for Hades Izanami, leading to her laying in slumber for an unknown amount of time.
Saya, now possessed by Hades Izanami, awakened at the end of the Continuum Shift phenomenon event. She seized Takamagahara with the assistance of Nine the Phantom, Hazama, and Terumi, shortly encountering her siblings and Noel Vermillion afterwards. Despite the reunion, it was kept short as Izanami took little interest in any of her foes. 
Still under the influence of Hades Izanami, Saya’s feelings towards her family still remained unchanged, as even Izanami admitted that her attachment towards Ragna was proving to be a nuisance to keep under control. Despite this, she still remained under Izanami’s firm control – even her body had changed as it was not affected by Hakumen’s time killer technique, which would kill most mortals.
- Evergreen Eyes
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 094
- BlazBlue: Continuum Shift, Story Mode, True Ending