how to get the good and perfect ending
hi, thanks for playing. There are only 2 endings (at this time) a good and a bad. To get the good ending you'll want to make sure you have um... stretched enough (you'll need to go to the adult store), also keep studying and exercising. Other than that, look for Lucifer's reactions to the choices you make (love points). You should be able to figure it out from there :) You can always go back to your previous selection and choose another one.
Also check out your picture gallery (in the phone) 
. There are 25 CGs to collect there. A good ending should get you 24 out of 25 of those. There's 1 image from a bad choice (although you could still get a perfect ending even when choosing that if you keepup the good choices after).
I don't want to give away too much in an open forum but you can DM me on twitter https://twitter.com/incubusgame1 and I'll give you a personalized walkthrough :)