As far as I’m concerned with most games I’ve played that dealt with taking care of slaves, it isn’t easy for them to break that habit, especially if they develop intense loyalty to you. They perceive you much higher than others because of your kindness and grow the desire to serve you, and probably only you.
Oh, and Ryoko is kind of just… content with the current master-servant relationship she has with Ryo (p.s: default name), so why bother changing it?
Well she was never a slave the necklace placed on her was just to protect her from those that hunts her race for their magic abilities, "MC" stated that more than once for him she is not any too and not wanted to be any of that stating from start that she is his partner(girlfriend) and their relationship is simply like this, so as i see there is no reason she use "Master" word. As Phoenix said too i agree to that, i dont mind if she will use "MC name", she sometimes use it without the "master".
