Tonan no Tsubasa

Twenty-seven years have passed since the previous king died in Kyo-kuni. Without a king, public order has broken down, and demons roam the land. Concerned about the deepening chaos in the country, a 12-year-old girl named Shujo makes a fateful decision. If the adults lack courage, then I shall be king!
(Source: Shinchosha, translated)