Mysterious Heroine X Alter is an anti-hero hailing from Servantverse-a quirkier version of reality where servants are alive and independent of their myths. She is an "anti-anti-Saber weapon," a tool forged to take out her nemesis, Mysterious Heroine X, hunting down Saber-class servants. Fueled by the Alter Reactor with calories, especially those coming from chocolate, X Alter deploys her dark holy sword Necro Calibur to create a mix of lethal power and comical eccentricity. She's an antagonist born from the darkest of concepts, Rounds of the villainous Dark, and struggles with her erased memories, a desire for revenge, and her odd quirks, such as her love of sweets and partnership with a malfunctioning robot called Black Knight.

Her story combines intergalactic battles, humorous rivalries, and a reluctant journey toward self-discovery, making her a unique blend of menace and relatability in the Fate universe.