Fate/Zero

Years ago, a young Kiritsugu lived on Alimango island in the Philippines with his father Noritaka and his father's assistant, a local girl named Shirley who called him Kerry. Noritaka was conducting research on immortality using plants. Despite being told by the townsfolk to stay away from Noritaka, Shirley is convinced his research will help the world and tells Kiritsugu she hopes he grows up to be great man. But one morning, Kiritsugu's father asks him if he went into his lab, and then tells him not to leave their house. Kiritsugu ignores his father's warning and goes to search for Shirley, only to find her at her house in a frenzy, devouring live chickens. Shirley reveals that she went into Noritaka's lab and use his immortality formula on herself, which turned her into a Dead Apostle vampire with an incessant craving for blood. Shirley begs Kiritsugu to kill her while she is still herself, but he is unable to do it, and she attacks the townsfolk, turning them into Ghouls. As chaos spread across the island, Church Executors and Mages from the Mages Association arrive to stop the outbreak and cover up the incident. Kiritsugu is saved from the Ghouls by a mercenary named Natalia Kaminski. She explains she is there to eliminate the magus responsible for the outbreak, which Kiritsugu realizes is his father. He confronts Noritaka over his work and its consequences, but his father simply shrugs them off as an accident, and tells Kiritsugu he plans to continue his experiments elsewhere. Angered at his father's response, Kiritsugu kills him. Emotionally broken and with nowhere to go, Kiritsugu agrees to escape together with Natalia.