Demon Slayer: I Became Muzan's Younger Sister

After transmigrating to the world of Demon Slayer, Ya Kui becomes Muzan’s younger sister. She hopes to use their bond of blood and love to change him, but an unexpected bout of amnesia leaves Muzan as a “filthy burden on his family.” Everyone still despises and rejects him, and he eventually becomes a test subject for medicines intended for his sister.

And so the siblings both become demons, yet set off down two entirely different paths. Ya Kui holds fast to her humanity and, in an era before the Demon Slayer Corps has grown strong, turns her broken body into a blazing fire to slay demons.