She traveled to Japan to participate in the Holy Grail War as a Master. Ilyasviel has an angelic appearance and unsurpassed potential as a Master. She lives in a castle on the outskirts of Fuyuki City, accompanied only by her Servant and two maids. Ilyasviel asks those she feels comfortable around to call her "Ilya". Her mother, Irisviel von Einzbern, fought alongside Saber and Emiya Kiritsugu during the previous Holy Grail War. In the Heaven's Feel scenario she wears what is called the Dress of Heaven, what was worn long ago by her ancestor Lizleihi Justica von Einzbern.
Much like the characteristics of all Einzbern homunculi, Illyasviel has crimson eyes and long silver hair. Similar to her mother, Irisviel, Illya also bears a genetic resemblance to her ancestor, Justeaze Lizrich von Einzbern, who was responsible for the formation of the core of the Holy Grail. She wears a collared, purple long-sleeved shirt with a light purple scarf to match, a long white skirt and purple boots.
Though Illya was born through natural means, her body has been heavily modified before and after her birth to maximize the magical circuits in her body, making her the ultimate homunculus and a refined vessel for the Holy Grail. As a result of the modifications to her body, her physical body develops slowly and her father, Kiritsugu Emiya, correctly assesses that her physical body will not develop secondary sexual characteristics. By the time she is eight years old, Kiritsugu notes that even his Thompson Contender is heavier than Illya is. A decade later, when Illya meets Shirou Emiya, she still looks as though she's a young girl rather than a young woman.
Illya is cheerful, being up-beat and generally having a positive attitude. She is outgoing, being friendly and easy to talk to. She is playful, being a bit mischievous. Deep inside, Illya is lonely, as she has no friends her own age to play with and is often stuck inside the Von Einzbern castle honning her magic skills. When she first meets Shirou, she sees him as something of an older brother. Though he is distrustful of her at first, he comes to like Illya, seeing her as a little sister.
Illya's interest in Shirou later appears to be romantic. In reality, she is the daughter of Shirou's adoptive father, Kiritsugu Emiya, and Irisviel Von Einzbern. This would make her his adoptive sister.
It can be said that Illya gets her outward personality from her mother. Like her mother, she is kind and childlike. however, she also has a bit of her father in her, as she can be cold and kill others without remorse.
She is in reality a homunculus, an artificial human given life by applying sorcery to sperm cells, originally created and raised by the von Einzbern family. She is actually slightly older than Shirou, her childlike appearance being a result of a distorted growth rate. During the Heaven's Feel scenario, it is hinted that the sperm used to create her was donated by Emiya Kiritsugu (it is confirmed in Fate/hollow ataraxia and the novel Fate/Zero), making her his biological daughter and Shirou's step-sister. Her personal interest in Shirou seems conflicted, due to the fact that Kiritsugu, whom she had loved as her father, remained in Japan as Shirou's guardian after betraying her family by destroying the long-sought Grail. The von Einzberns then trained Ilya harshly in order to groom her as the next vessel for the Grail, and a Master who would finally bring the Grail to the von Einzbern family. Ilya, with her mother dying during the Grail War and her father abandoning her shortly after, became angry with Kiritsugu, and consequently, Shirou. She feels that Kiritsugu abandoned her for Shirou, and is jealous of Shirou for that. However at the same time, she also feels very close to Shirou for his relationship with Kiritsugu.
The von Einzbern, one of three families of sorcerers involved in the Holy Grail War (the others being the Tohsaka and the Makiri/Matou), were responsible for creation of the vessel which the souls of the defeated Servants would go into, while the Makiri created the system of Masters and Servants and the Tohsaka supplied the land which became Fuyuki City. As such, Ilya was prepared by her family to act as a living vessel for the Grail.
Ilya has an unlimited supply of mana, thus allowing her to control Berserker without too much difficulty. Also, being the vessel of the Grail, she is able to wear the Dress of Heaven. Her body is capable of holding an immense number of Magic Circuits, and she also possess the Mystic Eyes of Binding.