The priest of Fuyuki Church and a complex antagonist driven by a nihilistic desire for thrilling experiences. Initially an informant, he becomes a direct threat in the final act, clashing with Shirou both ideologically and physically. His death marks the climax of the war.
Sakura is under Kirei's medical care, relying on his expertise to manage her Crest Worm condition.
Rin and Kirei interact within the church setting, navigating overlapping duties and moral dilemmas.
Kirei performs medical procedures on Sakura and provides critical diagnostic information to Shirou.