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

Known as the Holy Maiden of Winter. Illustrated in a stained-glass window in Einzbern Castle. Justeaze was the head of the Einzbern family two hundred years ago, and she originally devised the plan for the Holy Grail War system. She is also a homunculus, conceived to fulfill a special function related to the Holy Grail. Irisviel and Illyasviel were created based on Justeaze and can be thought of as successor models of Justeaze.

Fate/Zero Animation Guide I: Glossary of the holy grail war I