Secret NTR, the best kind of NTR!
It's hard to say what the game needs since it's so early, but a couple of things I'd like to see.
Consistent poses: Right now the game has an idle pose and it quickly swaps to other actions whenever the mentor does something. I think it'd be nice if he was always there, idle, and it just changed his animation and naturally returned to the initial pose instead of hiding the book, repositioning the GF, placing the mentor and then playing an animation.
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The last one here even changes the POV to behind them, and all of that makes the actions less "casual" than they could be, which would be way better.
This is surely going to be added in the future but a progression system is really important. Make it so every time they do something it makes them more willing to do that and more later, so the girl is corrupted further and they get bolder. If I'm getting it right, they won't do certain things if the torch light is above a certain value, maybe as time passes and the MC gets more and more focused/oblivious to them that threshold grows? If you add more than just "move forward" and "use item" to exploration, you can have a lot of fun events with all of that.
Also, for some reason, this game uses up all of my GPU. If you could tone that down it'd be great.
I'm excited to see what you will cook next. Godspeed, Dev!