Mysterious Heroine X is an Assassin-class Servant Universe Servant summoned by Ritsuka Fujimaru in the Grand Orders of Fate/Grand Order.

Also known as Heroine X (ヒロインＸ, Hiroin X?) and just X, Mysterious Heroine X is an unknown heroine who tasked herself in eliminating Saber-class heroines during Back Alley Satsuki - Chapter Heroine Sanctuary. Bearing the code name "A-X", X was a stranger who came from the "mysterious dimension" called the Servant Universe. Seeking to accomplish her "true, dearest wish" to "defeat all Sabers", she brandished Secret-Calibur and left her home town, leaving behind an "expression full of bitterness."

(Source: TYPE-MOON Wiki)