Reika Rikudou

Master of Assassin of Black during the Great Holy Grail War, Reika grew up without an actual family, and she suffered from abuse before being adopted. She was unable to do anything for herself during her childhood, and eventually became a prostitute simply because she needed money to survive. She becomes involved with the Holy Grail War at the age of twenty-three upon encountering Sagara Hyouma.

(Source: Type Moon Wiki)