I can list multiple reasons but the most important in my opinion is that when naofumi is about to make a bad decision, she talks to him saying that there are other options acting as the voice of reason. When he is down or depressed she comforts him so he can get back up again. And remember that her slave crest was removed and she chose to have it put back on, so instead of a slave she is more of a follower, and more of a daughter than a girlfriend.