My inspiration for this idea was Shamiko from 'Machikado Mazoku'. I am not looking for her to be the demon girl played in this story, but something similar to her.
Anyway, the story focuses on a teen boy who has rather severe social issues. Few or no friends, no luck with girls, not ugly, but not exactly attractive either. Digging around libraries/the internet, he comes across an ancient ritual. Designed to irreversibly sell your soul to a demon, but in return to have great fortune for the rest of your life, and possibly the next if you impress the demon enough.
In sheer desperation, and not actually expecting it to work, he went through the ritual late one night. Drawing an elaborate magical circle on the floor, chanting several incantations, and pricking his finger and letting a few drops of blood fall into the center of the circle. To his surprise the circle glowed intensely for a moment, a sign it did what it was meant to. There was no response from the demon in question, he had just bound himself to. The documentation that came with the ritual instructions said it would find him in the next couple days.
What he did not know is he had just sold his soul to a classmate, a young girl who did not know, until now, that she had demon blood in her. In truth her mother had been barren, desperate beyond description for a child, she had done a variation of the ritual, offered her body and gave in to sex with a demon to have a child. She never told anyone this, but her daughter was conceived through this.
The girls blood and power remained dormant, it might have remained dormant her entire life, but this ritual unknowingly targeted her as she slept that night. She was the closest 'demon'. The boy's blood and soul given to her triggered an awakening, activating dormant power and transforming her to have some visible signs of demonic blood. In the morning she would wake to find this transformation. Maybe maybe not finding out part of the reason from her mother, and before long finding out about the ritual and who did it. Perhaps sex is involved out of pity, or perhaps it helps enhance the girl's power.
This could go a lot of ways. Like with the inspiration source, it will probably be at least mostly on the lighter side, but as it involves demons I am not against it exploring darker aspects at times. The girl's power could add a lot of unusual things as well, including some of the tags mentioned above. If there is gender bending, I figure it would be of a temporary sort. We can discuss the details.