I noticed that your question asks how to write an introduction to a lesson plan, and does not ask for the introduction. So I will explain how. The purpose of the introduction is for those who see the lesson to understand your objectives. This could be for an outside audience, like an administrator or sub, but it could also be for you. You may return to the lesson after not teaching it for years, and the introduction will help you understand your purpose in writing it.
First of all, I would describe the reasons you are focusing on characterization and why you chose this book. One of the things that make this book ideal for teaching characterization is that it has richly developed characters, but there are very few of them. The Giver and Jonas are the most developed characters, but there is some characterization of Fiona, Gabriel, Mother, Father and Asher. By having students analyze all of these characters, you can teach the difference between dynamic and static characters and the difference between round and flat characters.
Next, I would briefly describe some of the activities that would take place during the lesson. Giving this overview will enforce how these lessons introduce and explore the concept of character.