Kuzuki Souichirou

A teacher at Shirou's school. He handles Contemporary Social Studies and Ethics.
Though he is Caster's Master, he is no magus, just a normal person. He suddenly showed up in Fuyuki about two years ago, under mysterious circumstances. The head priest of Ryuudou Temple took a liking to him, and he has been living at the temple ever since.
When he said that he's “just a killer that has rotted away” in the game, he wasn't lying. He really did commit
a murder in the past. As such, his hand-to-hand combat skills were not something granted to him by Caster, they were his own abilities.
Just as Caster felt affection for Kuzuki, Kuzuki too found meaning in Caster. In a way, you could say this living corpse's reason for fighting resembled the protagonist, Shirou's.
Also, it seems that he used to be a member of a group similar to the Nanaya mentioned in Tsukihime.
If there's ever a continuation of Fate/stay night, Kuzuki is the number one character I'd like to expand on.

Fate/side material: Fate Encyclopedia