I would write a topic sentence that includes several descriptive words about Scout. Then, I would choose to focus on one aspect of Scout, use a quote that demonstrates it and then spend 2 sentences interpreting and analzying that trait.
Next, transition into another trait, finding a quote about it, and do the same, analyze and interpret for two sentences. Finally, I would conclude by tying those two topics regarding Scout together.
Consider these aspects of Scout's character: moral, tomboy, creative, imaginative, considerate, defensive, naive, intelligent, or caring.
Since the content of an expository paragraph should inform or explain by using background information, you should have no trouble picking several examples of Scout's behavior and then explaining how or why they exude her personality. You might want to focus on her behavior during her first days at school; or you may want to select several scenes with Jem, Dill or Calpurnia. Certainly, there are quite a few examples of her tomboyish behavior as well as her independent and intelligent nature. She even has her loving moments, such as at the end when she finally gets to meet Boo Radley.