Three recommendation: 1) make the drain energy event more consistent. It's a bit annoying to savescum so that the event appear each day. Maybe make the event more frequent depending on the personality (if you plan that the demoness has different personality depending on your choice).
2) Allow the drain of more than 1 level. Again, maybe tie this to the personality of the demoness.
3) Not really a recommendation, more like a possibility: allow a domination loss path (a classic in level drain scenario). If the demoness becomes higher level than the Hero, she becomes more overbearing. You could also make that the enemy level are based on the highest level between you and the demoness. So, if get overtaken by the demoness, she starts leading the battle and you become more useless in them since you become underleveled. Though, if you do create this path, I would prefer that there is 2 possibilities: a playful version and a dominatrix version. In the first version, the demoness is overbearing but still cares for the hero, it's closer to roleplay when they get horny; in the second, she just treats him like trash all the times (I was never a fan of that but quite a lot of people like it)