Illyasviel von Einzbern, known as Illya, is a homunculus crafted by the Einzbern family to function as the Holy Grail’s vessel in the Fifth Holy Grail War. Born from Kiritsugu Emiya and Irisviel von Einzbern—participants in the Fourth Holy Grail War—her body and magical circuits underwent extensive modifications before and after birth, amplifying her power at the cost of halting her physical growth. Though chronologically older than her adoptive brother, Shirou Emiya, she retains a childlike appearance, a consequence of her conditioning to preserve her role as the Grail. Following the Fourth War, the Einzberns confined Illya to their castle, nurturing bitterness toward Kiritsugu for his apparent abandonment. Unaware of his desperate attempts to reclaim her, she was deceived into viewing him as a traitor. This grudge extended to Shirou, whom she pursued as Berserker’s Master during the Fifth War. The strain of sustaining her Servant’s power eroded her vitality, inflicting constant pain and accelerating her mortality. In *Heaven’s Feel*, Illya transitions from adversary to ally. After Saber Alter destroys Berserker, she collaborates with Shirou and Rin Tohsaka, exposing Angra Mainyu’s corruption of the Grail from the Third War. She aids in forging the Jeweled Sword of Zelretch to confront Sakura Matou, now a secondary Grail vessel. The True Ending sees her clad in the Dress of Heaven, a relic channeling the Third Magic, as she sacrifices herself to enact an incomplete Heaven’s Feel. Transferring Shirou’s soul into a new body and sealing the tainted Grail, she embraces her identity as his elder sister. Her outcomes diverge across timelines. In *Unlimited Blade Works*, Gilgamesh slays her to claim the Grail, while the *Fate* route depicts her joining Shirou’s home post-Berserker’s defeat, bonding with him like siblings despite her fleeting lifespan. Her demeanor merges playful mischief with lingering solitude, a reflection of Einzbern isolation and manipulation. Though driven by vengeance, traces of her parents’ compassion emerge in her guarded loyalty to Shirou. *Fate/Zero* explores her early years with Kiritsugu, whose affection for her clashed with his austere exterior. The Einzberns’ enhancements, designed to optimize her Grail utility, left her body frail and emotions fractured. These alterations irrevocably tied her destiny to the Grail’s cycle, anchoring her pivotal, sacrificial acts across intertwined narratives.