Kirei is a tall, imposing man with a sharp, dignified appearance that reflects his role as a priest and mediator. He has short, slicked-back black hair, dark brown eyes, and a perpetually calm and unreadable expression that masks his dark inner thoughts.

He is most often seen in his clerical attire, consisting of a long black cassock with a clerical collar, emphasizing his role as a priest in the Church. This stark, minimalistic outfit contrasts with the morally complex and sinister nature of his character. During battles or more intense situations, Kirei is occasionally seen wearing a black combat suit that allows for greater mobility, hinting at his proficiency in martial arts and his readiness for physical confrontation.

Kirei Kotomine is a character defined by his inner contradictions and moral ambiguity. Outwardly, he is calm, disciplined, and composed, traits befitting his position as a priest and supposed impartial overseer of the Holy Grail War. However, beneath this facade lies a man consumed by emptiness and a fascination with human suffering and despair.

Kirei struggles with his inability to find fulfillment or meaning in traditional ideals such as faith, morality, or love. Instead, he derives a perverse satisfaction from witnessing chaos and destruction, particularly when it results from others pursuing their ambitions or ideals. This dark nature is evident in his manipulative actions during the Holy Grail War, where he sows discord and works toward his own cryptic objectives while pretending to act as an impartial mediator.

Despite his twisted nature, Kirei is highly intelligent, a masterful strategist, and deeply philosophical. He often reflects on the nature of good, evil, and human desire, making him a complex and thought-provoking antagonist. His interactions with other characters, particularly Kiritsugu Emiya in Fate/Zero and Shirō Emiya in Fate/stay night, reveal his disdain for idealism and his belief in the futility of heroism.

Fate/Zero
Kirei begins the Fourth Holy Grail War as the Master of Assassin, working under the guidance of his mentor, Tokiomi Tōsaka. Despite his initial loyalty, Kirei is drawn to the enigmatic Archer, Gilgamesh, who recognizes Kirei’s dark nature and encourages him to explore his true self. Following Assassin’s defeat, Kirei forms a contract with Archer and ultimately betrays Tokiomi, killing him to pursue his own ambitions. Kirei’s evolving relationship with Kiritsugu Emiya serves as one of the central conflicts of Fate/Zero, culminating in a climactic confrontation.

Fate/stay night
During the Fifth Holy Grail War, Kirei assumes the role of overseer, a position meant to ensure fairness and impartiality in the conflict. However, he uses this position to manipulate events behind the scenes. After killing Bazett Fraga McRemitz, Kirei takes control of Lancer (Cu Chulainn) and continues to conspire with Gilgamesh, who remains bound to him from the previous war.

Kirei serves as a major antagonist across all three routes of Fate/stay night. His philosophical clashes with Shirō Emiya reveal his belief in the futility of ideals, while his actions, such as corrupting the Holy Grail, drive much of the story’s conflict. His final battles with Shirō in the Unlimited Blade Works and Heaven’s Feel routes showcase the depth of his villainy and his resolve to pursue his own twisted desires.

• Gilgamesh (Archer)
  Kirei’s most significant ally, Gilgamesh, acts as both a partner and a philosophical confidant. Their mutual disdain for humanity’s flaws strengthens their bond.

• Tokiomi Tōsaka
  Kirei’s mentor during the Fourth Holy Grail War, whom he ultimately betrays to pursue his own path.

• Kiritsugu Emiya
  A major foil to Kirei in Fate/Zero, Kiritsugu represents a contrasting ideal of utilitarianism and sacrifice, which Kirei finds both fascinating and repugnant.

• Shirō Emiya
  In Fate/stay night, Kirei becomes an ideological opponent to Shirō, challenging his idealism and forcing him to confront the complexities of human desire and morality.

Kirei Kotomine is one of the most iconic antagonists in the Fate series, celebrated for his philosophical depth, chilling demeanor, and pivotal role in shaping the events of both the Fourth and Fifth Holy Grail Wars. His character serves as a reflection of the series’ themes of morality, human desire, and the nature of heroism. Kirei’s actions and ideology leave a lasting impact on the Fate universe, solidifying his place as one of its most memorable and compelling figures.