Most are based on your story choices.
Not all are unlockable, it has been a while.
Cue the list:
Chloe
* Original > Default (?)
* Bimbo > After progressing the ORG branch of the main story to a certain point, ask her to change personalities in her room. (interchangeable with Original)
Gwen
* Damaged > With Yandere Mode turned on in preferences, ignore Gwen for an entire month (over 30 days of no sex).
Jessica
* Auntie > Agree to call her 'Aunt Jessica'
* Adulteress > In her love level 4 event, pick the right option (HINT: Look up what 'adultery' means)
Luna
* Commoner > After getting her to love level 4, pick the option to humiliate her.
Pixie
* Cocksleeve > Have sex with Pixie on her normal size.
* Schooled > After becoming the owner of the college, teach a class during a weekend.
Qarinah
* Dreamgirl > After reaching love level 4, the event may trigger when sleeping.