Sitonai

An Alter Ego-class High-Servant & Pseudo-Servant.

Identity
A Pseudo-Servant in the body of Illyasviel von Einzbern and a composite High-Servant.
The Saint Graph's registered True Name is Sitonai. This is due to the fact that it seems like Sitonai is the one who manifests the strongest outside among the three Heroic Spirits (Divine Spirits) gathered inside the yorishiro.
The person who became the container for these Divine Spirits is not a human Mage, but the latest and the final Homunculus created by the Einzberns. One possessing Command Seals that cover the entire length of her body, first-class precision in Magecraft and the best performance when it comes to being a Master. The ability to function as the Lesser Grail is also concealed in that body.

(Source: TYPE-MOON Wiki)