Mirai Kimizuki is Shihou's younger sister, and she's been hospitalized after contracting the Apocalypse Virus as a child. Her condition is pretty much the driving force behind everything Shihou does—he refused her repeated requests to end her life and instead dedicated himself to finding a cure. Since civilian hospitals couldn't treat the virus, he eventually joined the Japanese Imperial Demon Army specifically to access the military resources needed to save her. Before that, the two of them survived together in the monster-infested outside world, but they eventually made their way into Shibuya.
Content compiled by AnimeList.moe from publicly available sources.
