I see you have good taste. To become her slave you need to choose the right dialogue options during her tea parties which occur every time you fight her (you must lose to her twice to get all three tea parties). Individualistic, rational and shy are her preferred dialogue options. At the end of the third tea party she will offer you slavery if you were deemed compatible enough. Choose the option that gives +100 stress and enjoy. Also, if you were deemed very compatible, you will get a special one-time scene with her after accepting to become her slave.