Justeaze Lizrich von Einzbern

The Einzbern family head who became the foundation of the Great Holy Grail over 200 years ago.
Just like Illya, she was not a proper human, but was instead a magic circuit born for the sole purpose of becoming the Great Holy Grail's core.
In fact, Illya, who was born from the union of a Justeaze-type homunculus and a foreign magus, could be considered her latest model.
As for Justeaze herself, she still exists within the center of the Great Holy Grail. The circuitry covering the surface of the crater in the cavern is actually her magic circuits, enlarged and multiplied.
These magic circuits, almost a kilometer in diameter, are microcosm of the universe created from a single human body.

Fate/side material: Fate Encyclopedia