Those who have read some of my posts likely already know. Before coming to the Nexus, Saeko lived in a small desert city, and a monster destroyed her city, killing most of the residents. Including her big sister, Aoi, who died protecting Saeko.
For the sake of spoilers for a future plot of my own, I will not continue. But for prying eyes, I shall say Saeko is hiding something. There is more to this story than what she lets on.