Tohsaka Nagato

The Tohsaka family head during the very first Holy Grail War, two hundred years ago.
He took part in the ritual at the request of Makiri Zouken and Justeaze.
Though traditionally the Tohsaka were a clan of secret Christians, one day a strange old man appeared out of nowhere and led them astray. Ever since then, they have devoted themselves to the world of magecraft.
With their contacts in the Holy Church and favorable treatment from the Association as disciples of a magician, they are in an enviable position.
It is due to their connections with both organizations that they were appointed the managers of Fuyuki.
At any rate, Nagato viewed magecraft and martial arts as equal, and was attempting to reach the Root via a state of mushin (no-mind) when he was recruited by the Einzberns and Makiri.
As you might expect from Rin's ancestor, he was prone to goofing up. Also, it seems that his daughter was the one with talent for magecraft, and ended up being far more useful in the construction of the Great Holy Grail than her father was.

Fate/side material: Fate Encyclopedia