Kimetsu no Yaiba: Not by Blood

When Kesha is presented to his new adoptive sister, he instantly swears he will protect her. And he does, when a demon goes rampant in their small, mountainside village four years later. Kesha is able to protect Katta, but the massacre changes her. She is no longer the joyful child, she quietens and sobers up.

Katta then goes on to become a Demon Slayer along with her brother, killing demons, rising quickly in the ranks. However, there is a bubbling fury in Katta's heart that no killing will cool. She wants to end it all. So she and Kesha agree to go to the root of the problem: Muzan Kibutsuji.

But demons are not what Kesha expects at all, not after an encounter with a fellow demon slayer and the little sister he carries in a box.

But will he be able to show Katta that? Will they be able to win against Muzan Kibutsuji and his Uppermoons?
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Demon slayer does not belong to me, neither do the canonical characters.