Kirei Kotomine's 80 year old father, Risei is a priest in the Church and the regulator of the third and fourth Holy Grail Wars. A friend of Tokiomi Tōsaka's, Risei actively supports him and conspires to ensure that Tokiomi will obtain the Holy Grail in the war, despite the fact that as a member of the Church, he should be ensuring only someone who is worthy of the a powerful artifact like the Grail should be obtaining it.

While he is proud of his son, who has demonstrated himself to be a perfect heir, Risei fails to understand Kirei in any sense. He believes that his son, who has recently lost his wife of two years, may be able to find some purpose in life again by aiding Tokiomi during the Holy Grail War.

Both Risei and Tokiomi, upon discovering that Kirei's command mantra had appeared three years before the start of the war, both came to believe that Kirei was destined to provide Tokiomi with an additional Servant and associated command spells for the upcoming war. Without informing Kirei as to the true nature of the mark that appeared, they transfer Kirei from the Church to the Mages Association as Tokiomi's pupil. Neither man reveals to Kirei that he can also obtain the Holy Grail, instead telling the young man that he is performing a mission for the Church.

(from Wikipedia)