An apprentice of Lord El-Melloi II, she is a girl who lived in the countryside of Britain before moving to the city. Originally a normal villager, she is revealed to have been altered since birth as a vessel for the second coming of Artoria Pendragon, the Saber-class servant of the Fourth Holy Grail War to wield the Holy Spear Rhongomyniad. Born as a result of her village's attempts, her wielding the spear results in her appearance to slowly starting to transform into that of Artoria's. After El-Melloi II realizes who she is, he requests that Gray keeps her face hidden under her hood.
Wikipedia