Each girl will have the following, though they may start at different stages depending.
Attributes:
Libido
Inhibition
Morality
Confidence
Sexuality
Sexual Skills:
Oral
Upper body (includes hands, breasts and more obscure things like elbows)
Lower body (includes vaginal, anal and again less obvious things, like thighs)
Feet
The main attributes will each each range from 1 to 10, the higher being the stronger the feeling. During certain events, suggestive ideas can be signalled to her to encourage her to do certain things. Roll checks might be made against these stats to see whether or not she'll go through with them or not. If not, points may be used to force her to do it anyway, though any skills gains from this method won't be as great and the greater the roll fail, the more points it will cost. Every time she passes a roll check, she has a chance to gain a point in that attribute. Critical failures (worst possible roll) will lose a point.
Morality refers to her willingness to intentionally commit a crime or harm someone else - the higher the stat, the less likely she is to do it.
Sexuality will work on a scale instead, with 5 being bisexual, 0 being totally straight and 10 being totally gay. The principles behind increasing and decreasing the value are the same, depending on which way you're trying to push her.
The skills all range from 0 to 100. Different scenarios will increase them at different rates. For example, watching a blowjob video might increase her oral by D5 whilst actually giving head might increase it by D10.
That's pretty much most of what I have for now. None of this is necessarily set in stone and I'm open to suggestions. If possible though, I'd like to try not to make it any more complex than it is right now. If we've learned anything from the Ero-Empire threads, I think it's that if something appears to be complicated, it drives people away.