Note: The female lead is an overpowered young lady who flirts with everyone everywhere. This is not a hot-blooded battle story, the characters are heavily out of character, and there are multiple pairings, though romance makes up very little of the story. All pairings at the end of this story have both partners alive. Don't ask how they survived, they have a cheat.
Zero, is it really you, Muzan?
The Demon King’s throne belongs to whoever is strong enough to claim it. If you can sit on it, why can't I?
I, Kannonzaka Genki, was “born” at almost the same time as Muzan. Luckily, I was a step ahead of him. But I never cared about that. All I cared about was freedom. My father developed the so-called medicine, but Muzan killed him. Oh, well, that works out nicely for me. What a way to repay the man who saved your life, by chopping him down in a single stroke. Now I can't go outside and see the light.
But it doesn't matter. That man used me as a test subject for the medicine, all so he could cure Muzan. If he's dead, he's dead. After a thousand years of roaming the world and taking in its sights, I've finally returned to my homeland. My dear little brother, big sister is coming for you~
Come on!!