Before she became a member of Sector 6, I am guessing Yanagi was a spec force squad commando that venture into the dangerous areas to protect New Eridu from threats. One of those is a war that has broken out between humans and Oni. It was a cold night before what would be an assault by Special Divisions that would wipe out the Oni for good, Yanagi ventures into the Oni village solo to for a specific mission to be carried out before dawn came. While near the village she for the first time met a young Soukaku whom she ends up protecting from an explosion hit the both of them. Yanagi was taken to have an audience with the current leader of the Oni where her and her general debated on killing her or using her as hostage until Yanagi woke up and spoke her peace about how their leaders failed to stop the war from happening and how it was their inaction that lead the war to continue costing lives on both sides. The general was having none of it blinded by his hatred was told to leave by the leader along with her other soldiers and allowing Yanagi to explain why she came on her own. Yanagi tells the leader of the Oni of the previous leader's demise and that there was no need for the war to continue. The leader confirms this and that she is carrying the burden left behind by what her father had created knowing she holds the fate of her kind on her hands. The leader is uncertain whether she can bring herself to make that choice and as such Yanagi presents her the one chance to do so and they place their trust on one another. The oni leader who has been battered and wounded from many battles offers her life to Yanagi to make that choice she can no longer make. Yanagi carrying with her the future of the oni leads the village to salvation. She takes responsibility of the oni allowing them to become citizens of New Eridu as they intended. To this day Yanagi has upheld the oath she had made and even took the responsibility of raising Soukaku as her own. To think Yanagi and Soukaku being roommates carry a deeper purpose.