Mirai Kimizuki is the younger sister of Shihou Kimizuki. She was infected with the Apocalypse Virus despite being a child and is being treated in a hospital. Shihou took care of her, even though they lived in the outside world with monsters. She wanted Kimizuki to kill her, but he refused and they eventually entered Shibuya. He also joined the Japanese Imperial Demon Army in order to gain access to the military resources that are needed to cure her as the virus cannot be treated in civilian hospitals.

(Source: Owari no Seraph Wikia)