The Ubuyashiki Family had a daughter nobody liked. Her name was Chiyo.
One day, she happened to meet an older brother nobody liked, either.
He later gave himself a new name: Muzan.
And eventually, he became the boss everyone in the Infinity Castle knew about, a piece of trash whose name made male demons fall silent and female demons weep. She, meanwhile, became the one and only troublemaker who could keep that boss in check.
Some Six Eyes guy: Damn it, I wanted to become a demon to get stronger, but now I can't even walk in the sunlight...
Troublemaker: Great, come work for us. Room and board included, plus healing care. Three treatment sessions and you'll be completely satisfied.
Some Rainbow Cult leader: Oh, beautiful miss, your obsidian eyes have captivated me at first sight...
Troublemaker, mercilessly: Enough. I'm not going to endorse your Paradise Cult.
Some dojo member in prison: Is this really all my life can amount to...?
Troublemaker: Don't worry, I'll get you out.
Some strongest hanafuda swordsman: Miss, would you care to go bask in the sun with me?
Boss of Trash: Wait, Chiyo! Have you forgotten what we are?
Troublemaker: Uh... I guess it's not entirely out of the question?
The heroine is Muzan's younger sister, and the hero is Douma. A story about fighting monsters, leveling up, reforming demons, and everyone going to see the sun together.
Theme: Do not fear fate. Challenge it bravely.
