Mirai Kimizuki

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.

[Spoiler] She was one of the children that was being experimented on by the Hyakuya Sect to create a weapon of mass destruction, as part of the 'Seraph of the End' plan in destroying the world. Guren knew of this beforehand, thanks to intel provided by Mahiru nine years ago. [/Spoiler]

(Source: Owari no Seraph Wikia)