Kenzo noticed her trying to leave, so he tackled her to the ground. Beelithina didn't give up easily, but she realized Kenzo had serious business hence why he was so desperate for the recharge that came with sex. Then, something strange happened to her body.
A surge of energy, unlike anything she'd felt before, pulsed through her. It was the kind of energy she struggled to obtain from humans, but Kenzo provided it abundantly.
But she wasn't the only one that benefitted from this, Kenzo noticed that he could use her body as conduit to convert his energy into dark energy and reabsorb it through sex.
It was simple: Kenzo, a human, needed dark energy obtained through rituals, but he couldn't convert the natural energy humans had. On the other hand, Beelithina, a demon, needed the natural energy humans possessed.