With perhaps the single exception of Bob Ewell, almost all the characters in this story show some kind of passion or love. Atticus shows it for his children and for his fellow man (through his defense of Tom), Scout shows it through her actions and her approach toward Jem, Dill, Calpurnia, and Atticus, among others. Cal shows it by trying to bring up the Finch children according to high standards, Aunt Alexandra shows it by trying to transform Scout into a little lady, and so the list goes on. It is more difficult to find characters who do not display intense feelings or passion in this novel -- Bob Ewell may be the only one who we can say does not show passion, but even his courtroom outbursts and his malice toward Atticus show a form of passion.
I think that Arthur (Boo) Radley showed passion at the very end of the book when he saved the Finch children from the vengeful Bob Ewell.
Atticus and Heck Tate showed passion when they made the decision not to bring Arthur Radley out into the public eye as the town hero even though he deserved the honor.
Atticus showed the greatest passion and love of his fellow man in defending Tom Robinson in court. He stood on his values against overwhelming odds in his hometown. "Greater love, has no man, than he would lay down his life for his friends." (John 15:13) Atticus laid his career on the line in defense of Tom Robinson. He sat at the jail house at night knowing that a lynch mob would show up, yet he stayed right there.
Scout showed courage when she confronted the entire lynch mob by the recognition of one of her classmate's fathers. After a brief conversation, the lynch mob dispersed. Scout loved her father and knew that something bad was about to happen, and she had to do something about it. She showed genuine kindness and neighborly love when she walked Boo Radley home after the heroic scene of saving the children's lives.