Illyasviel von Einzbern

A semi-heroine. Berserker's Master.
In the story, she was a mysterious girl who approached the still-oblivious Shirou while calling him “big brother.”
She possesses innocence and cruelty in equal measure, and for some reason is interested in Shirou.
The kind of character that makes you feel like angels and devils might just be the same thing.
Her power as a Master is the greatest in history.
With a command spell engraved over her entire body and a magic circuit that overwhelms that of normal magi by an enormous margin, there is no doubt that she is an existence born for the sole purpose of winning the Holy Grail War.
Though she played a key role in every route, her true colors were only revealed in the last one. Until then, she was just a worrisome child that got along well with Shirou despite coming to Japan to kill him.
During the plotting stage, she actually had her own route.
However, if we had tried to make it, it would have delayed the release of Fate/stay night by another half a year. Rest in peace, Illya route.

Fate/side material: Fate Encyclopedia
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The daughter of Irisviel and Emiya Kiritsugu, and one of the heroines of the Fifth Holy Grail War.
…That's not a lie, dammit! The Illya route exists in all of our hearts!
Anyway, though there vague insinuations about it even in Fate/stay night, in Fate/Zero the shocking matter of Illya's age was revealed right off the bat.
As for why Saber didn't show Illya any sort of leniency in the fifth war, that's because she had never actually asked Iri what her daughter's name was, and she thought Illya looked far too childish to be the same girl—she assumed that the girl she had seen riding on Kiritsugu's shoulders ten years ago would have developed into a voluptuous women like her mother by then. So, she must have seen Illya as just another unrelated Einzbern homunculus.
…Shut up, it's not a lame excuse!

Fate/Zero material: Fate/Zero Encylopedia
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