In dictionary order:
Plant Alarune: You touch her while you're naked, high chance of this one instead of one on top of leaf.
Red dress ork: Random from what I can see.
Fox girl: Becomes available around chapter 3, but no idea how I triggered it.
blue dress ork: 2nd can be gotten by not getting up after an H. Lily one is by being in female form while she's naked (becomes available around chapter 2, girls previous met will not get updated)
fire spirit: 2nd can be gotten from the ones you find later inside the protective field. 3rd one can be done from giant one in one of the locked in areas where you have to kill a certain number of enemies to escape.
First boss: Touch her in female form.
Plump witch: Can't really recall how to get it.
Huge boob elf and loli elf (?): haven't found yet.
Ballerina: After first H scene, get up fast and press down while she's still lying down.
Flower Alarune: Gotten a bit later once you're able to get back to this area in chapter 3, after her flower H scene....immediately raise up lust and she will go into 2nd scene.
Not-sure-what-she-is Goth Loli: haven't found yet.
Doppelganger?: haven't found yet.
Eye patch girl: haven't found yet.
Twin lolis: haven't found yet.
Slime girl: Gotten from giant slime you can find starting chapter 3.
Cat girl idol: Might need to let her do first scene first before second appear, not really sure.
frog girl(?): I have no idea how I triggered this one.....it just happened when I was trying to get out of the H lock she kept me in.
Dark skin loli elf: Stand just outside her hop's distance and she'll try to trip you. If she does so successfully and hops on you, she'll start 2nd one.
Succubus: no idea.
Necromancer?: After her first scene, she'll give you a debuff. Get caught by her again while debuff is up.
Big breasted elf: No idea.
Purple slime: Can be gotten while she's in her final form.
Bunny girl loli: After first h scene, initiate another by pressing down.
Invisible gem girl?: No idea, but sure making her visible plays a role.
Twin tail loli with blue rings: 2nd get caught by another girl with her nearby, she'll start peeing. Get up and get close to her to trigger. 3rd one, I'm not exactly sure how to trigger. It just sometimes happen.
Goth loli fox loli: 2nd one is actually easier to get than first. For first, hold down when she floats towards you. Easiest way to get this is to trigger first, knock her off and then walk over to where she's going to land.
Pretty much how far I've gotten in game, not sure how to get to the area after successfully delivering the girl to the snow lady.