Pixie. There are probably hundreds of descriptions of them, depending on the folklore you follow they can look like small humans (with or without elf ears) or something hideous.
You are correct that sex is used as a tool by some. But you are wrong about Li, sex is something she wants because she enjoys it, but not having sex with her doesn't prevent doing business with you. Of course, she is upset if you reject her but that's a normal human feeling to have in that situation, but she isn't that petty for the rejection to ruin something potentially beneficial for her.
And this bit... everything you fancy. Some have a faster payout, some slower. Depending on what is the best position to have it in the story, some paths have less content right now but it changes because it makes more sense to have them more visible than the other.
There is options built-in that most people will most likely miss, such as slower progression with Miyuki, including an option of rejecting her completely. That is harder to get intentionally because she is actively trying to get into your pants. With some characters, you can fuck up completely with the first choice you get, they still appear in the story but the dialogue will be different when it needs to be. Sometimes with choice A, it is irrelevant if you have sex with some character, if you chose option B it is beneficial to sleep with that character instead.
And all of these will influence how things end. It's all planned ahead in a big spider web of decisions that will drive the uninitiated insane upon looking at it.