Kotomine Kirei is the overseer of the Holy Grail War and resides at the church on top of the hill. He is callous and sarcastic, but also quite knowledgeable. As the supervisor of the Grail War, it is his duty to uphold the rules of the Holy Grail War, cover up any incidents that might expose the War to the public, and give sanctuary to any Masters who drop out. Both Tohsaka and Shirou appear to detest him, though in truth they don’t seem to dislike him too much. Despite the enmity between the Mage’s Association and the Church, Kotomine is a member of both organizations, which is presumably the reason he was chosen to be be the overseer.
Kotomine plays an important role in the unfolding events in all three routes, but the route that features him most heavily is the final one, Heaven’s Feel where he takes an active role early on.