Foreigner-class version of Mysterious Heroine X. She has three distinct designs depending on her Ascension level:
Mysterious Heroine XX wields Twinmyniad, a double-bladed version of Rhongomyniad. Her breasts get slightly larger with each Ascension, though never as big as Artoria Pendragon (Lancer).
Personality-wise, she's an older, more mature version of Heroine X. She's nowhere near as obsessed as X is with eliminating all the other Sabers, to the point that she sometimes forgets about it completely.
This tag implicates artoria_pendragon_(all) (learn more).