Kotomine Kirei

A supervisor dispatched by the Association and the Holy Church.
He secretly operated as the Master of Archer in the previous war, and as the Master of Lancer in the current one.
In his early twenties, he was selected as a participant in a Holy Grail War supervised by his father. Afterward, under the guise of “being dispatched” from the Church, he entered the Association.
As one of the only survivors of the Fourth Holy Grail War, he inherited the role of supervisor.
He is qualified as an executor of the Church, and he specializes in spiritual healing.
Of course, despite being a first-class executor, he is still no match for the seventh member of the Burial Agency.
His excellent attack power against spiritual bodies is a testament to his unwavering (albeit distorted) faith.
Though he seems to enjoy destruction, he is actually a magus of “creation” just like Shirou. His ability to heal spiritual and mental injuries is at the level of a priest.
Wicked, but not a villain. Depraved, but not cruel.
He is the ultimate antagonist of Fate/stay night
If Archer is an existence that shines light on Shirou's distortion, then Kotomine is a dark wall that brings it into stark relief.
His initial concept was “somebody you'd know is the mastermind behind everything the instant you met him.”

Fate/side material: Fate Encyclopedia
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Fate/Zero's other protagonist. When I saw the young Kirei next to Kiritsugu in the material book that came with first print editions of Fate/stay night, with his short hair and earrings, well, my heart turned somersaults in my chest. It's part of what inspired me to write Fate/Zero in the first place.
To write him, I just got rid of the older Kirei's state of enlightenment and composure, and replaced them with hesitation and inner turmoil. In Zero, when he engages in self-introspection his memories of the past are distorted, especially in regards to his wife. Accordingly, the more philosophic self-analysis from Fate/stay night is probably a better reflection of his true inner self.
In the first draft, I was a bit too cognizant of his height from the original story, and kept indiscriminately using words like “gigantic,” “hulking,” and “towering” to describe him. As a result, I was called in by Type Moon
and chastised for misrepresenting Kirei's image, and his height ended up casually being changed.
Originally he was taught baji quan by his father, Risei, as a form of spiritual improvement, but through his blood-drenched experiences as an executor it was gradually sublimed into a warped version specialized at destroying the human body. Needless to say, the fact that he was so far off from his father's ideal form of kung fu was probably just one more thing he could not forgive himself for during his youth.

Fate/Zero material: Fate/Zero Encylopedia
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Without needing to be introduced, he is the man who appears as the villain in Fate/stay night and Fate/Zero (though his mentality is quite complex). He doesn't appear at all in these novels, but hints of his existence are shown here and there, so he has an entry here.
Kotomine Kirei is living as an extremely respectable priest in Fuyuki even now. The Fourth Holy Grail War didn't occur here, so he didn't shift his focus to his own nature. It might be a different story if he participated in one of the subspecies Holy Grail Wars, but he'll never have such a chance as long as he is in Fuyuki.
In other words, he is still in anguish over the meaning and karma of his existence even now.
He heard about who his older brother Shirou is from his father Risei, but they have rarely interacted with each other. This is not done on Kirei's part, but rather because Shirou consciously avoids him.
This is because Shirou noticed the “distortion” within his stepbrother Kirei. Of course, he would very much like to free Kirei from his anguish, but no matter how he thought about it, it wouldn't end well… Additionally, if the one-in-a-million chance occurred where an event caused him to affirm his own distortion, Shirou was greatly worried that the first person Kirei would target would be him.
It would be like pouring gasoline on a volcano that's on the verge of erupting even now. Thus, Shirou kept his distance from Kirei as much as possible from the beginning to end of their relationship.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia