She is an assistant of Lord El-Melloi II at the Mage's Association's Clock Tower. She first met Lord El-Melloi II when he arrived in her hometown. She became his apprentice, and was admitted to the Clock Tower of London.
She has a striking resemblance to Saber, which Lord El-Melloi II believes is because she's a distant descendant of King Arthur. Her face brings unpleasant memories of the Fourth Holy Grail War for him, so he tells her to wear a hood to hide her face.
Gray has possession of the Noble Phantasm Rhongomyniad, the holy lance of King Arthur, that has been passed down through the generations by preserving its mystery through a similar manner to the Tradition Carrier trait held by the Fraga family.
Gray also possess a unique Mystic Code called Add.
She is about 15 years old in the events of Adra Castle Case. In Fate/strange fake, she appears in her early twenties. In Fate/Grand Order, she's introduced as an Assassin-class Servant in the Case Files collaboration event.