Why is Curzon's friendship so important to Isabel? How does she change as a result of knowing him?

Isabel's friendship with Curzon is a friendship that is constantly developing throughout the book.  When she first meets Curzon, there is little indication that Isabel will eventually put herself in great danger to rescue him from prison.  When they first meet, Curzon makes a stop in a store and comes out with two rolls.  He gives her both and claims that he is not hungry.  It's not until much later in the story that Isabel learns the truth. In chapter 36, Isabel visits that very same store and learns from the owner that Curzon "pointed to you out the window . . . Told me you were likely to die from hunger if I didn't help" 
It's clear to readers that Curzon has a heart for helping Isabel.  He gets her those rolls, and it is him that gets Lady Seymour to help nurse Isabel back to health after being branded.  Curzon continually places himself at risk to help Isabel because he genuinely values her friendship.  She is more to him than another potential spy.  Isabel eventually comes to understand that she needs Curzon's friendship.  He has helped her from the beginning, and that friendship is so important to Isabel that she risks her own escape from the Lockton's home to free him from prison.  

My remembery called up the feeling of being locked in the stocks, of my face being burnt, of him watching me from across the courtyard; him watching out for me. 'Twas Curzon that made sure I survived. 'Twas he who had been my steadfast friend since the day they brought me here. 
[...]
I couldn't. No—not couldn't. I shouldn't. But I had to. I had a debt to pay.