Kirei Kotomine (言峰 綺礼, Kotomine Kirei) is the main antagonist of Fate/Zero and Fate/stay night. He acts as the Master of Assassin during the Fourth Holy Grail War, he then later forms a contract with Gilgamesh after Assassin’s defeat. Their contract lasts into the Fifth Holy Grail War, and he also deals a fatal wound to Bazett Fraga McRemitz to take control of her Servant, Lancer. While he is an ordinary Master during the Fourth War, he acts as the supposedly impartial mediator during the Fifth War.
The Black Keys are simple tools used by us expert executioners. But that was not what I was referring to, take a closer look at the picture, do the lines not seem simple? For I drew this earlier to show that using a mediocre tool like Paint can be applied to create something skilled. Much in the same for humans, a man who wields a simple weapon can be deadly if he is skilled with it.– Kotomine Kirei - https://www.facebook.com/FatherKotomine