Hazel, is a shy, loving person who loves to read. She is also smart. Augustus is a very strong, outgoing, and nice person. As you can tell these characters are polar opposites with their interest, yet they change each other and grow to love each other. 

Some of the themes in this book are love: Green is trying to tell the message that love can be between anyone no matter conditions they might face. Gus loves her he decides to use his wish on her. Gus takes Hazel and her mom to Amsterdam to meet Hazel's favorite author who turns out to be a jerk. Courage: The characters also show how much courage it takes to battle cancer.
One of my favorite themes in this book was to be yourself, Hazel and Gus have to find who they really are. With the help of each other and family they do just that.  Hazel at the beginning of The Fault in Our Stars doesn't know who she is. All she knows is that she is sick and loves to read. Augustus has more self awareness but not much. He has many friends who he communicates with so the reader knows more about his personality. That's why character development was so important in this book. Green did an amazing job of showing how much you can change as a person and how it affects not just you but the people around you.
Hazel realizes she needs to take more control over her life. She stands up to her parents because she realizes it's time for her to start living like a teenager. Gus know that he doesn't have much more time. He makes the most of his final day and spends them with Hazel and in the hospital. What was so important to this is that even though Gus is weak he still makes time for what important to him, and I think that's a message Green is trying to tell us.
Overall The Fault in Our Stars was an book with many messages and an amazing storyline. I would recommend The Fault in Our Stars to lover of C.R.F and readers who want characters that challenge themselves and discover who they are. Definitely a thumbs up.