Let's see... gonna give a bit of feedback:
First of all let's talk about the blue demon, I wouldn't call her evil but she is more in the egoistic side of things, overall she seems to try to influence us to do things that we consider morally wrong for her own benefit, for example the first night that we met her not only did she force us to have sex but was influencing us to cum on the other characters of the house. One thing to note is that if we refuse and decide not to do it she will comment about how our mind is stronger than she thought.
Based on that she seems the manipulative type, so perhaps she needs some kind of stat to show that situation and how much control she has on us and having an impact related to our ability to refuse certain situations. Like for example if she has enough influence she could force us to do the "cum on the people in this house thing" without being able to refuse.
Related to it rather than normal sex scenes she should see what your character fetishes are and take note of it, in fact she could use that information so that you would have sex with her on the night by appealing towards your fetishes.
Following that line of thought... wouldn't it be kind of nice that the MC could develop fetishes while interacting with characters depending on his actions?.
Let's take Natalie foot scene as an example, depending on our choices we could manifest a fetish towards foot stuff and even a degradation/submissive fetish depending on if we keep the play to the end.
But this is just a line of thought that could be interesting to follow mainly because it would show us a character that likes what we like. Related to the use... it depends, from being more excited in certain situations, not being able to refuse other situations and things like that, overall it could be interesting.
To summarize what I said in my feedback is:
1º The blue demon feels like she could have more potential related to her manipulative side, in fact having her learn our fetishes and tempting us to have sex using them feels quite good, even more that her level of power/influence over us had an effect on our character or game style.
2º That the game tracks our actions and we could get fetishes that could change or influence our actions. It would be a nice addition that the blue demon used them to tempt us.
But well, this is just my personal opinion, overall the game mechanically is kind of normal, go to x place and click x character to get a scene that advances the storyline. On that point feels a bit generic, the story itself seems that it could be fun, overall it would be interesting if the blue demon could end up having as much control as her sister's had on her slave.
Good luck with the game and have a nice day
.