The answer to this question can be found in Chapter 6, which is when Isabel discovers what Curzon is really doing apart from being a slave, and Curzon tells her about his work for the rebels and also how they will help anybody that assists them by spying for them, particularly through granting freedom to slaves. This chapter is very interesting because it explores the feeling of loyalty, as Isabel feels loyal to her owners because they give her food and provide her with shelter. However, Curzon argues that this loyalty Isabel feels is misplaced, as she is not actually viewed as a human being by Master Lockton and his family. This actually will work in Isabel's favour, he argues:
You are a slave, not a person. They’ll say things in front of you… ‘Cause you don’t count to them. It happens all the time to me.
Curzon therefore promises that Isabel, if she helps work as a spy for rebels, will be supported and helped as she seeks to gain her own freedom.