Much like the other Einzbern homunculi, Illyasviel has pale skin, crimson eyes, and long white 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 Magic 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 his Walther rifle is heavier than Illya is.[11] A decade later, when Illya meets Shirou Emiya, she still looks as though she's a young girl rather than a young woman.