Atrum Galliasta

The Master of Caster is the unnamed Master of Caster in the Fifth Holy Grail War of Fate/stay night.

Unlimited Blade Works (Visual novel)
The Master of Caster does not appear in the original visual novel, only being told of briefly in Caster's flashback to her meeting with Souichirou Kuzuki. One month before the start of the Fifth Holy Grail War, he summoned Caster as his Servant. He was noted to be a legitimate magus in his late thirties who had a medium build and few other characteristic of note. While he dreamed of victory, he had no will to fight and wished to simply wait for the other combatants to kill each other. He did not trust Caster originally, treating her coldly due to being inferior to her as a magus, limiting her magical energy to a level below his own, and insulting her for being inferior to the other Servants. Caster, giving up on obtaining victory with him within a few days, played the part of an obedient Servant to fill her Master's ego, placating him to have him spend his Command Spells on "meaningless things." He was made to believe that she would be faithful even without them, but she killed him the instant he used his third Command Spell. Spiteful of the contract still binding them together, she killed him using Rule Breaker.