Question: who is mysterious heroine x?
Answer: Assassin's True Name is Mysterious Heroine X (謎のヒロインＸ, Nazo no Hiroin?), also known as Heroine X (ヒロインＸ, Hiroin X?) and just X, an unknown heroine who tasked herself in eliminating Saber-class heroines during Back Alley Satsuki - Chapter Heroine Sanctuary.