Kiritsugu Emiya hailed from a secluded island where his father’s reckless magecraft experiments unleashed a zombie outbreak, culminating in the island’s ruin. Orphaned and rescued by a compassionate mage named Shirley, he found fleeting stability until her fatal experiment drove him into the tutelage of Natalia Kaminski, a mercenary who honed his combat prowess and instilled a ruthless pragmatism: prioritize maximum survival through cold calculus. As the freelance mercenary "Magus Killer," Kiritsugu specialized in eliminating magi who preyed on the vulnerable, blending firearms with tactical magecraft to dismantle tradition-bound foes. A mission to eradicate a vampire forced his hand to sacrifice Natalia, downing her aircraft to prevent a pandemic—a harrowing choice that solidified his utilitarian creed. Desperate to abolish conflict, he entered a secret war for the Holy Grail, a wish-granting artifact, alongside Irisviel, a homunculus he wed. Their alliance saw sabotage, betrayals, and unflinching violence. Upon securing the Grail, it offered a genocidal "solution" to peace, which he rejected, destroying the relic and igniting an inferno that consumed a city. From the ashes, Kiritsugu saved Shirou, a lone survivor, raising him while concealing his bloodstained history. Ravaged by guilt, the Grail’s curse, and failing health, he devoted his final years to nurturing Shirou’s idealism. His estrangement from his daughter Illyasviel, enforced by her clan’s vendetta, stranded her in isolation and resentment. Kiritsugu’s legacy rests in paradox—a visionary who spurned omnipotence to cling to fragile humanity, a strategist who traded love for survival, and a broken mentor whose failures and convictions etched an indelible mark on Shirou’s quest for justice.