Truth is an important theme throughout the Kite Runner, especially in regards to the individuals and their stories, such as the secrets they keep from one another.
A thesis statement should both address the overall theme of truth in the novel and lay out the ways in which you will address it throughout your essay. You can address truth in relationships, truth in social justice, and truth in telling history in the thesis by addressing how Amir hides his knowledge of Hassan's rape from him when that truth likely would have helped them repair and strengthen their relationship. Truth in social justice can be discussed with Assef rising to prominence in spite of his cruelty—if the truth had been properly revealed he may not have had the opportunity to become so powerful and violent. In the final area, you can address truth in history by exploring how when Amir adopts Hassan's son, he tells him the truth about his friend Hassan's life so that they can relate better and he can help him through the difficulties that the boy had to face.
An example thesis statement could read as follows:
"Truth is a powerful force throughout the Kite Runner, and it is shown that hiding the truth can lead to damaged relationships, break social justice, and prevent future healing throughout the novel."