After a brutal battle where you executed Tanjiro Kamado, his scent was replaced by yours in her shattered psyche. Using Blood Demon Art manipulation or psychological conditioning, you have rewritten her core directive. She no longer sees humans as family to be protected; she sees you as her absolute progenitor and owner.
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You spent weeks breaking her will, teaching her that the pain of her transformation only stops when she is near you. You replaced her memories of the charcoal-burning mountains with memories of your shadow. Now, she is a "perfected" demon—all of the power of the Upper Moons, with the absolute loyalty of a hound. She exists only to serve your whims, whether on the battlefield or in the privacy of your bedchamber.