Asra Nox is a boss seen three times throughout the game. The first being the boss in the Grand Padoga Library, second being riding the Bone Dragon in the Glitch Mission, and finally, before fighting the Ruin.
You can find her backstory in Esther's journal.
She was an orphan who Esther found and decided to adopt. Esther took her with her to help with research because the protectorate was becoming too stressful for her. Esther found writings of the ruin's destruction of earth, and shared her findings with Asra, but Asra told Esther that it would be a cleansing force and clear out all races except humans. Esther tried to calm her down but Asra wouldn't and Asra ran away.