It really kind of bothers me that he hasn't just freed her already.  I know that it might hurt her pride or whatever, but it's kind of fucked up to me that someone with our modern sensitivities is willingly keeping a slave.  Doesn't it ever cross his mind that their relationship could be even better if they were equals?  And now with him getting a second slave, and just the general way all the slaves seem happy and healthy, it just seems like this manga is saying slavery is great.