She was a member of the Baneful Bloodline and a master of classical martial arts. She had also taught herself her own "mortal combat" skills. She was murdered by Eva before the events of OneeChanbara ORIGIN.
Although Tsubaki doesn't appear in the game, Saki has mentioned multiple times that Aya reminds her of their mother, suggesting that some aspects of Aya's personality resemble Tsubaki's.