A young Teutonic aristocrat who travels to Japan to participate in the fifth Holy Grail War as the Master of Berserker. Illyasviel has an angelic appearance and an unsurpassed degree of magical power, seemingly able to control Berserker with ease. She lives in a castle on the outskirts of Fuyuki City, accompanied only by her Servant and her two maids Leysritt and Sella. Illyasviel asks those she feels comfortable around to simply call her "Illya". In the prequel, Fate/Zero, it is established that Ilya is the daughter of Kiritsugu Emiya and Irisviel von Einzbern and is therefore Shiro's elder stepsister, a fact which causes Shirou incredible grief when he realizes Kiritsugu sacrificed a life with her for him. She is not a human, but rather a homunculus, created to serve as the vessel of the Holy Grail. When wearing the Dress of Heaven and empowered by the deaths of at least six Servants, she is capable of performing a limited version of the Third Magic, Heaven's Feel (天の杯（ヘブンズフィール） Hebunzu Fīru?), one of the Five types of True Magic (魔法 Mahō?) which can accomplish impossibilities beyond modern science or sorcery.
In the Fate route, she lives peacefully with Shirō after her Servant is defeated, although none of them are aware that she will soon die, her homunculus' body having been designed to survive only up to one year after the Fifth Holy Grail War's conclusion. In the Unlimited Blade Works route, she dies weeping over the dying body of Berserker, her eyes having been slashed and heart torn out by Gilgamesh and is comforted by Berserker, who created a thirteenth life for himself through sheer force of will, as they die together. She has a greatly expanded role in the Heaven's Feel route, in which her background as Kiritsugu's daughter is revealed. She eventually sacrifices herself to save Shirō and close the Great Holy Grail in the True End as she embraces her role as Shirou's "big sister" rather than the "little sister" role she played in Fate and for most of Heaven's Feel. Her ultimate fate is not revealed in the Normal End, although she is presumed to have perished due to her homunculus' design a year after the Fifth War's conclusion.
Originally Ilya was planned to be one of the heroines and possess her very own route branch in which Shirou would forgo his ideals to save her; however the idea was scrapped during development due to time and monetary constrains. IIya's route was instead combined with "Heaven's Feel", a fact which is shown to cause Ilya no small amount of frustration in the Tiger Dojo scenes in the visual novel.