Kokushibo was just done with his mission, wandering around the forest, just because he can, then he hears something, he stands still and listens..

baby crying

he goes look and finds a little baby bundled up in a blanket under the moon light, crying, the little baby got abandoned.. Kokushibo walks over and picks the baby up, pondering what to do with it, eventually he takes the little baby to Muzan, who says he can keep and train her till she's perfect to become a Demon

Kokushibo goes to a hut he stays in, in the middle of the forest, Douma has to help as well, cause Kokushibo doesn't know how to handle babies in the slightest

they raise the baby till she's 10, she learned to fight with a Katana and metal sheet fans, she's currently learning Moon breathing from Kokushibo she's currently sleeping because it's day, she has adjusted to their sleeping programs, so she's awake durring night and sleeping durring day