Note: Illya is 18. For those unaware or just never knew. In Fate/Zero she was 8/9 and it takes 10 years before the next Grail War 8/9+10=18/19. She's half Homunculus as well which complicates her growth... That and Fate series does confirm having immense mana in the body does indeed complicate things. It's why her hair is white from the stress of it. Prisma Illya throws her age and everything on it's head... That and Illya circuits were sealed at birth, so her hair isn't white like her FSN self.
Illyasviel von Einzbern (イリヤスフィール・フォン・アインツベルン, Iriyasufīru fon Aintsuberun?), often referred to as Illya(イリヤ, Iriya?), is the Master of Berserker in the Fifth Holy Grail War of Fate/stay night. She is the daughter of Kiritsugu Emiya and Irisviel von Einzbern, both whom participated in the Fourth Holy Grail War, and the adoptive older sister of Shirou Emiya.
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. A decade later, when Illya meets Shirou Emiya, she still looks as though she's a young girl rather than a young woman.
I can write both versions of Illya, discussions in Messenger will decide which one I do! She'll be Futa with females and I'm open to writing Kuro as well as Miyu~ Her Servant version Sitonai is up too.