Shirou Kotomine

"Servant Ruler, Amakusa Shirō Tokisada. Do I resemble someone else? A mere coincidence."

- Shirou in Fate/Grand Order

"Once before he was an existence that I had to defeat. However, now that he’s an ally, I don't hold any grudges... Even so, it’s a complicated feeling, and I think Amakusa Shirou probably feels the same way."

- Sieg talking about Shirou in Fate/Grand Order

Shirou Kotomine, true name Amakusa Shirō Tokisada, is the Ruler-class Servant of the Einzbern Master‎‎ in the Third Holy Grail War, the leader of Red Faction, and the main antagonist of Fate/Apocrypha. He is also the archenemy of Sieg and Jeanne d'Arc in Fate/Apocrypha.

Taking on the name Shirou Kotomine (シロウ・コトミネ, Shirō Kotomine?), he becomes the Master of Assassin of Red in the Great Holy Grail War. He later contracts with Caster of Red, Archer of Red, Lancer of Red, Rider of Red, and Caster of Black.

Shirou also appears as one of the Servants able to be summoned by Ritsuka Fujimaru in the Grand Orders of Fate/Grand Order. There, he appears as a supporting character more often.

He is voiced by Kōki Uchiyama in Japanese version of the anime series and throughout the Fate series, and by Max Mittleman in the English version of the anime.

Backstory
A Japanese Christian that led the Shimabara Rebellion, Shirou was born in the Edo period, he was a boy of miracles who could nearly be called a saint. How exactly he was discovered, no one knows.

A good half of his lifetime is wrapped in mystery. He, who concentrated on his studies since childhood, began to perform many miracles at the frontier. Having healed wounds and walked on water, he eventually began to be enthusiastically worshiped as the son of God by the peasants that believed in a prohibited religion.

In Fate/Grand Order
Shirou also makes an appearance in Fate/Grand Order, being the same Shirou from Fate/Apocrypha and has memories of the Third Grail War and the Great Holy Grail War. He debuted for the 2016 White Day event and is a major character for Christmas 2016. An alternate timeline version of him appears as an Avenger in Shimosa chapter, being an assistant to Caster Limbo.

Interlude
Shirou's interlude was titled Let Us Tell a Tale of World Salvation. In this interlude, Shirou accompanies Ritsuka Fujimaru and Mash Kyrielight to prevent even the smallest Singularities from emerging. Shirou tells his past to Ritsuka and Mash, before they detected a polluted Holy Grail, the black mud oozing out can contaminate Servants. Together, they defeated the remnants of Zouken Matou and his insects. Then, Angra Mainyu pops out from the mud, he asks who wants the corrupted Holy Grail. Shirou betrays them because he has a goal and battles Ritsuka, Mash and Angra Mainyu.

As Shirou was defeated, the Holy Grail was about to explode. Shirou insist that he'll destroy the Holy Grail because he was defeated so he'll put an end to this. As they return to Chaldea, Shirou informs the group that as long as there is a Holy Grail, he will claim it. However, to his surprise, Ritsuka says he'll stop Shirou as many times as it takes. He and Ritsuka cannot come to an understanding, but Shirou is willing to place his own dream under seal so he can fight alongside with Ritsuka.

As punishment for his earlier betrayal, Ritsuka orders Shirou to clean his room, to which Shirou would devote his all.

Trivia

 * In Chinese fandom of Fate/Apocrypha, the Chinese fans are Rooting for the Empire by considering Shirou to be the true protagonist of the story and like him better than Sieg, the story's real protagonist, to be awfully plain by their standards. This was mildly toned down after Sieg made his appearance in Fate/Grand Order with an improved characteristics.
 * Higashide said that not only readers, but even his friends retorted, "Really?", when they discovered his true identity. Higashide apologizes and calls this coincidence to be like destiny.