Illyasviel von Einzbern is a female mage who served as the master of Berserker in the Fifth Holy Grail
War. She's the daughter of Kiritsugu Emiya, who served the master of Saber in the Fourth Holy Grail War, and the adoptive sister of Shirou Emiya, who served as the master of Saber in the Fifth Holy Grail War. After the Fourth Holy Grail War ended, due to Kiritsugu betraying the Einzbern Family, they prevented Illya from seeing her father again to groom her to be a master in the next war, telling her that Kiritsugu abandoned her for Shirou, which obvioulsy wasn't true at all. Because of this, after Kiritsugu's death, she held resentment for Shirou, wishing to kill him in the war more than anything. She's a mage of expedential mana, despite her young age, which is required in order to control the Berserker Class Servant that she acquired.