Your thesis statement about being an outsider could reflect many of the people in the novel, To Kill A Mockingbird. If you are going to use three of the people as your body paragraphs, you will need to decide how each of them is an outsider. If you choose one of them and show the three ways they remain an outsider, pick the one you feel the most strongly about. For example, one thesis statement might be that, "Boo Radley was the person who best exemplifies being an outsider in the novel. His inability to meet people face to face, his reluctance to live among the community as one of them, and his extreme shyness about talking to anyone except children make him a perfect example of being the outsider." Now, other characters in the novel reflect being an outsider also; you just have to decide which character or characters you think are best and find the reasons why you think that. Then, you can construct your thesis and include examples from the novel. Remember that being an outsider can be different such as physically, emotionally, beliefs , religion etc.