What difficulties throughout the novel reveal the true nature of the characters in To Kill a Mockingbird?
It would be possible to answer this question from the perspective of several characters. I will use Atticus to illustrate your idea.
Atticus is an ageing single parent. He does have Calpurnia to help him with domestic chores, but he has the complex task of bringing up a son and a daughter with the beliefs and values he feels will make them good citizens. Scout is impulsive and curious. He teaches her to empathise in chapter 3-
"You never really understand a person until you consider things from his point of view—“
“—until you climb into his skin and walk around in it.”
Atticus shows his children in chapter 10 that he is capable of deadly force but uses it only when necessary. He has the strength and skill to defeat others, but only employs his skills fairly. This is when he explains to the children
"it's a sin to kill a mockingbird"
Atticus' greatest action in the novel is of course acting as defence fot Tom Robinson. He is aware that the trial will never produce a fair verdict, and that in some ways his efforts could be seen as futile. However, Atticus produces a fair, detailed and respectful defence, maintaining the dignity of himself, Tom Robinson and Mayella Ewell. It is fitting at the end of teh trial in chapter 21 that the courtroom stands to acknowledge Atticus' efforts to bring about unity, peace and dignity for all.
Atticus was never going to win the trial, but he was able to present the evidence as clearly as possible that Tom's conviction was about race, not deed. He manages to retain the dignity and respect the community have for him, whilst teaching an important lesson for all.