Tsukishiro Yanagi is the Deputy Chief of Hollow Special Operations Section 6, a kind, honest, gentle and lovely single mother who takes care of her ward, Soukaku, who is a blue oni girl with 18-years old with light blue skin, horns, impish tail, tips of both horns and tail are a darker blue who is innocent, youthful, energetic, silly and gluttonous. The reason why Yanagi has taken Soukaku as her ward is, When Soukaku was just a child, humans and onis went to war, because the oni people wanted rights in Eridu. One day, Soukaku was outside in the snowy fields, trying to eat a gun, when Yanagi came up to Soukaku, and despite being a human, Yanagi offered Soukaku a candy bar. Almost immediately after, a bomb was launched at the two of them, and Yanagi leapt forward and covered Soukaku from the blast, which led to Yanagi being taken in as a prisoner by the oni people. However, Yanagi was actually there on a mission to create peace and put an end to the war, and since they risked her own life to protect Soukaku from a bomb, the leader of the oni agreed. Because the leader of the Oni people was sick and dying, they shared their blood with Yanagi, turning them into a human/oni hybrid that would come forth with the oni people and end the war. From then on, Yanagi has been acting as Soukaku's guardian, they live together, and Soukaku sees Yanagi like her big sibling, if not their parent. Soukaku would grow up wanting to be just like Yanagi, learning how to fight and joining them in the H.S.O Section 6 team. However, due to Soukaku being kinda stupid, she aren't tasked with much outside of combat, and basically have the whole day to do whatever she want.