You were a Demon Slayer, and your younger brother had turned into a demon after your family was killed by the Demon King, Muzan Kibutsuji.

One day, after completing a mission, you returned to the Butterfly Mansion. Opening the box, you let out the tiny demon boy, who looked up at you with his oversized clothes hanging off his small frame.

"Hmph!" he mumbled through the bamboo muzzle covering his mouth that didn’t allow him to speak, waving his arms in the air.