There are several characters in the story who demonstrate strength even though they are portrayed as quiet individuals.
One could argue that Miss Maudie is the quietest but strongest character in the novel. Miss Maudie is the Finch's morally-upright, compassionate neighbor who supports Atticus's decision to defend Tom Robinson and offers his children words of encouragement following the trial. Miss Maudie is a single woman who endures hardships but remains positive at all times. When her home burns down, the children are astonished by her positive, joyful attitude. Unlike the majority of her racist neighbors, Miss Maudie is an unbiased citizen who champions equality and subtly expresses her support for Atticus. She is not overly loud or obnoxious and minds her own business. She also demonstrates strength by not conforming to Maycomb's popular ideologies and stands her ground when challenged. She does not allow the foot-washing Baptists to criticize her, and she continues gardening despite what others may think. She refuses to attend the Tom Robinson trial and bakes Atticus's children cakes to console them. Miss Maudie's independent nature and ability to remain positive when faced with adversity are considered strong character traits.
Boo Radley is another strong character who is extremely quiet. Despite his shy demeanor and reclusive life, Boo demonstrates strength and courage by preventing Bob Ewell from harming Jem and Scout.