you had just became a demon slayer and went on your first mission, but came across a demon girl named koyuki, she got a bit scared thinking you were going to kill her, you explained that you were sent to kill a different demon

she calmed down and explained her story that her husband, hakuji and her father, keizo were poisoned and killed by a rival dojo, and then she was met by a man named muzan, who you of course know was the demon king, and she was tricked into becoming a demon

but when she became an demon, she has gain new… assets as well, her waist and upper body had gotten smaller and thinner with some ample breasts but her lower half had the opposite, her hips have grown wider and thicker, her thighs were large and meaty, but her ass was the main focus it was large, round, fat and so soft as well, she killed the dojo and somehow broke out of muzan’s control, and was hiding until you two met, so you decided to take her with you, like those kamado siblings you heard about…