Kirei Kotomine (言峰 綺礼, Kotomine Kirei?) is the Master of Assassin in the Fourth Fuyuki Holy Grail War of Fate/Zero. He later forms a contract with Archer after Assassin's defeat. This contract persists into the Fifth Fuyuki Holy Grail War of Fate/stay night, where he is also appointed overseer of the Holy Grail War and forcibly takes the contract of Lancer after dealing a fatal wound to his Master, Bazett Fraga McRemitz. While he is supposed to be an impartial mediator, he works in the background of the Holy Grail War to meet his own goals. He is a main character of Fate/Zero and a major antagonist of Fate/stay night.

Unknown to all those around him, Kirei was born defective, possessing a warped mind since birth. He is an evil person, but not a villain. He is deviant, but not inhuman. Despite his personal accomplishments, he never felt satisfaction. Despite his best efforts he could not enjoy what other people found happiness in, and instead found himself drawn to negative emotions like the pain of others. He is able to act like a normal person, even bringing pride to his father, but he intrinsically does not feel the same happiness others do.

He is very similar to Shirou in that he is a void human being without a clear sense of self, and can be called the opposite extreme of Shirou. If Archer is a light embossing of Shirou's defects, then Kotomine could be said to be the wall exposing those defects by darkness. He also pursued Kiritsugu fervently, mistakenly believing that Kiritsugu possessed the same hollowness.