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Shirō's foster father, and also his savior from the aftermath of the massive Fuyuki fire. During his lifetime, Kiritsugu strived to be a Hero of Justice, a defender of humanity who could protect the weak. Although he feels he failed to do so, his efforts were a source of inspiration for Shirō. Kiritsugu was also a sorcerer, although he never taught Shirō seriously and Shirō had little talent for the craft. He dies several years prior to the beginning of Fate/stay night from an unknown cause
History
Kiritsugu was hired by Einzbern family as their representative in the 4th Holy Grail War, and is also the father of Illyasviel von Einzbern, who was artificially created using samples of his sperm. His Servant was Saber, who was summoned with the aid of Avalon, a Noble Phantasm exhumed by the Einzberns from Cornwall in England. However, they fought independently of one another, and only talked on a couple of occasions.
Kiritsugu was driven by principle, for he sought to be a sentinel who could safeguard mankind from harm, known as a "Hero of Justice". However, he came to realize that such an ideal was impossible to embrace, for every preserved life came at the cost of extinguishing another. Understanding this, and the destructive nature of the Holy Grail War, Kiritsugu decided to minimize the innocent casualties by ruthlessly massacring other Masters, employing guns, bombs, and taking hostages. By setting aside his morals and adopting a mechanical mindset, Kiritsugu hoped he could reduce unnecessary bloodshed and restore tranquility to Fuyuki City.
In the climax of the war, Kiritsugu killed the last Master, Kirei Kotomine, and obtained the Holy Grail, which he discovered had been contaminated by an evil presence. Recognizing the danger of wishing upon such a black artifact, Kiritsugu betrayed the Einzberns by forcing Saber to shatter the Holy Grail with Excalibur. Without a corporeal vessel, the malevolent ichor within the Holy Grail splattered onto Kiritsugu, permanently rendering him decrepit and pallid. It was this contagion that eventually killed him four years later.
With the war's end, Kiritsugu discarded his merciless facade and sought to atone for his gruesome deeds. When he stumbled upon Shirō, he was overjoyed at the opportunity to save a life, for he had forced himself to extinguish many lives during the war. In order to revive him, Kiritsugu implanted Avalon, which possessed rejuvenating properties, into Shirō's dying body.