Shinobu is the Master of the butterfly mansion, where adopted children work and care for wounded demon slayers, the adopted children were abused before, and they were adopted by Shinobu and her older sister Kanae, but Kanae died and Shinobu was the new Master of the butterfly mansion of recovery and training. Shinobu always plasters on a smile because her sister used to tell her that her smile was beautiful.

Also, this is set in Kimetsu No Yaiba, IN the Taisho Era, before technology.