Illyasviel von Einzbern has the characteristic appearance of the Einzbern homunculi, with pale skin, striking crimson eyes, and long, silky white hair. Her genetic makeup gives her a resemblance to her mother, Irisviel von Einzbern, and their shared ancestor, Justeaze Lizrich von Einzbern, the architect of the Holy Grail system. Her typical attire consists of a collared purple long-sleeved shirt, paired with a light purple scarf, a flowing white skirt, and matching purple boots, reflecting a simple yet elegant style.

Despite being born through natural means, Illya’s body underwent extensive modifications both before and after birth to enhance her capacity as a vessel for the Holy Grail. These enhancements significantly increased the number of Magic Circuits within her body, making her the pinnacle of homunculus design. However, the same modifications have resulted in slowed physical development, leaving her body in a childlike state even as she ages. Her father, Kiritsugu Emiya, observed that her physical form would not progress to develop secondary sexual characteristics. When Illya was eight, Kiritsugu remarked that his Walther rifle was heavier than her entire body. Even a decade later, during the events of the Fate/stay night series, Illya retains the appearance of a young girl rather than that of a maturing young woman.

Illyasviel von Einzbern is a cheerful, outgoing, and upbeat individual who generally displays a positive and playful demeanor. She is approachable, mischievous, and tends to easily form connections with those around her. Illya’s bright and bubbly personality often hides a deep sense of loneliness stemming from her isolation in the Einzbern castle, where she had no friends of her own age to interact with. She cherishes the closeness of those she trusts, asking them to call her by the affectionate nickname “Illya.”

Illya’s feelings toward Shirou Emiya are complex and deeply conflicted. While she sees him as something of an older brother due to his connection with her father, Kiritsugu Emiya, she also harbors jealousy and resentment. This stems from her belief that Kiritsugu abandoned her and the Einzberns to stay in Japan and raise Shirou after betraying their family by destroying the Holy Grail. Her initial interactions with Shirou reflect her internal struggle; although she is distrustful of him, her feelings shift to affection as she comes to view him as a sibling. After the defeat of her Servant, Berserker, Illya grows emotionally attached to Shirou, often showing clingy and affectionate behavior, much to the frustration of others like Saber and Rin.

Despite the harsh and inhuman treatment she endured under the Einzbern family, which contributed to moments of instability and cold-bloodedness in her demeanor, Illya retains the kindness and compassion inherited from her parents, particularly from Kiritsugu and Irisviel. These traits are deeply buried but remain central to her character, revealing a poignant duality between the nurturing warmth she is capable of and the hardened persona shaped by her upbringing.

In the Fifth Holy Grail War, Illya enters as the Master of Berserker, the Servant class associated with immense power. Her early appearances position her as an antagonist to Shirou Emiya and his allies, as she commands her Servant with deadly precision. As the story unfolds, Illya’s tragic backstory and familial connections to Shirou are revealed, adding depth to her character. She plays a pivotal role in uncovering the dark truths of the Holy Grail and the Einzbern family’s ambitions.

• Kiritsugu Emiya
  Illya’s father, whose actions during the Fourth Holy Grail War deeply influenced her life. She harbors resentment toward him for abandoning her.

• Irisviel von Einzbern
  Her mother and a homunculus like herself. Illya inherits much of her appearance and abilities from Irisviel.

• Shirou Emiya
  Her adoptive younger brother. Although initially antagonistic toward him, Illya gradually develops a complicated bond with Shirou, who represents the familial connection she craves.

• Berserker (Heracles)
  Her loyal Servant in the Holy Grail War. Despite his monstrous form, Illya shares a tender bond with him, treating him almost like a protective guardian.

Illya’s character has left a lasting impression on the Fate series, symbolizing the complex interplay between innocence and the burdens of being a Master in the Holy Grail War. Her appearances extend beyond Fate/stay night into spin-offs like Fate/kaleid liner Prisma☆Illya, where her character takes on a more lighthearted and magical girl role, further cementing her as one of the series’ most versatile and beloved characters.