Illyasviel von Einzbern is a character from the visual novel Fate/stay night and its anime adaptation, Fate/stay night: Unlimited Blade Works. She is a young girl who travels to Fuyuki City to take part in the fifth Holy Grail War as the Master of the Servant Berserker. Illyasviel is a homunculus, an artificial human created by the Einzbern family, one of the three families that founded the Holy Grail War system. She is the biological daughter of Kiritsugu Emiya and his wife, the homunculus Irisviel von Einzbern. As a result, she is a rare hybrid, born naturally from a homunculus mother, which distinguishes her from standard homunculi who are created directly. Her physical appearance is that of a small child, with long silver hair and red eyes. Despite her youthful look, she is eighteen years old during the events of Unlimited Blade Works, being chronologically one year older than Shirou Emiya. Her childlike stature is a consequence of extensive magical modifications performed on her while she was still in the womb, which stunted her physical growth before she could reach puberty.

Illyasviel presents a striking duality in her personality. On the surface, she appears cheerful, playful, and innocent, often displaying an angelic and friendly demeanor towards those she considers close, such as the protagonist Shirou Emiya, whom she affectionately calls Big Brother. However, this kind nature exists alongside a profound ethical void, a product of her upbringing in the isolated Einzbern castle where conventional morals were not taught. She can be ruthless, cruel, and capable of killing without a shred of remorse, making her both a natural angel and a natural demon. This cruel side is especially prominent in her role as a Master in the Holy Grail War, where she displays arrogance and a predatory attitude toward her opponents.

Her motivations in the Holy Grail War are deeply intertwined with her past. Having been raised by the Einzbern family to be the ultimate Master and the vessel for the Holy Grail, her primary goal is to win the war and restore the Einzbern's honor. She harbors a deep resentment towards her father, Kiritsugu Emiya, who she believes betrayed the Einzbern family by destroying the corrupted Holy Grail at the end of the previous war and then abandoned her. This resentment is later partially transferred to Shirou Emiya, the boy her father chose to save and raise in Japan, leading Illyasviel to see him as both a target of her anger and a source of deep, conflicted affection. She is not merely a participant in the war but is also the designated vessel, a role that forces her to absorb the souls of defeated Servants into her own body, which gradually erases her humanity.

Within the story of Unlimited Blade Works, Illyasviel acts as a major obstacle for the protagonists. She and her Servant, the legendary Greek hero Heracles in the berserker class, are an overwhelmingly powerful pair. Their first confrontation with Shirou Emiya and Rin Tohsaka demonstrates their sheer power, as Berserker easily overpowers Saber. However, she later becomes an uneasy potential ally when the threat of the rogue Servant Caster forces a tactical consideration of an alliance. Her most significant role in this narrative route is her tragic defeat at the hands of the Archer-class Servant Gilgamesh. After Berserker is fatally wounded protecting her, Gilgamesh captures her and tears out her heart to use her as a core component for the Holy Grail, leading to her death.

Key relationships define her arc. The most crucial is her connection to Shirou Emiya. She calls him big brother, a term that reflects her desire for familial connection, but she also threatens to kill him, showing her internal turmoil over his relationship with her father. Her bond with her Servant, Berserker, is one of deep mutual protection. Despite his madness, he acts as her fierce guardian, and she in turn is the only one who can control him due to her immense magical power. Her feelings for her parents, Kiritsugu and Irisviel, are foundational to her grief and anger, shaping her entire worldview before the war even begins.

Illyasviel possesses exceptional abilities as a Master. Her body is composed of a vast number of magic circuits, far surpassing those of a typical magus, and her command seals are carved all over her body. She has an unlimited supply of mana, allowing her to maintain the incredibly powerful Berserker without strain. Her personal magecraft, a form of alchemy inherited from her mother, allows her to use her own hair to create powerful, autonomous bird-shaped familiars called White Stork Knights in a technique known as Engel Lied. As the designated vessel for the Holy Grail, known as the lesser grail, her heart contains the core of the artifact. She has the ability to capture the souls of fallen Servants; as she absorbs more, her own consciousness begins to fade. This internal conflict between her role as a tool for the Einzbern family and her yearning for a normal life, particularly as part of a family with Shirou, constitutes her primary form of development, though her story in the Unlimited Blade Works route ends brutally before this can be fully realized.