Illyasviel von Einzbern is the natural-born daughter of Kiritsugu Emiya and Irisviel von Einzbern, heavily modified by the Einzbern family before and after birth to become the ultimate vessel for the Holy Grail. These modifications maximized her Magic Circuits but caused abnormally slow physical development, granting her a permanently childlike appearance despite being chronologically older than her adoptive brother, Shirou Emiya. She exhibits characteristic Einzbern traits: pale skin, crimson eyes, and long white hair, genetically resembling both her mother and their ancestor Justeaze Lizrich von Einzbern, who formed the core of the original Holy Grail. Her early childhood involved affectionate bonds with her parents, particularly her father Kiritsugu. After the Fourth Holy Grail War, the Einzbern family confined her within their castle, isolating her and imposing harsh training to prepare her as a Master and Grail vessel for the Fifth Holy Grail War. During this imprisonment, she was manipulated into believing Kiritsugu abandoned her, fostering deep resentment toward him and Shirou. Unbeknownst to her, Kiritsugu repeatedly attempted rescues but was barred by the family's bounded field, dying with regrets over his failure. As Master of Berserker (Heracles) in the Fifth Holy Grail War, Illyasviel initially targets Shirou, driven by jealousy and Kiritsugu's perceived betrayal in choosing Shirou after destroying the Grail. Her outwardly cheerful and playful demeanor masks profound loneliness and emotional instability from years of isolation, compounded by the life-draining physical agony of sustaining Berserker. Despite her vengeful intentions, she retains core kindness inherited from her parents. Her character evolves significantly during the Heaven's Feel route. After losing Berserker, she seeks refuge with Shirou, gradually forging a genuine sibling bond. Upon learning she is Shirou's older sister, she sacrifices herself by performing the Heaven's Feel ritual. This act severs Shirou's soul from his dying body, destroys the corrupted Holy Grail and Angra Mainyu, ensures his survival, and reunites her with her mother's spirit in death. In the Fate/kaleid liner PRISMA☆ILLYA alternate universe, her parents abandoned the Holy Grail War during her infancy, sealing her war-related memories and magical abilities. Raised alongside Shirou as a normal child in Fuyuki City under maids Leysritt and Sella, she exhibits a more innocent, shy, and cheerful personality. Contracting with the magical wand Ruby, she becomes a magical girl, navigating adventures unburdened by her main timeline's tragedy. This version actively cultivates friendships, displays a love for anime and cute things, and avoids the solitary existence of her original incarnation.