In To Kill a Mocking Bird, how does Atticus show integrity by acting reliably?

Throughout To Kill a Mockingbird, Lee continuously demonstrates the ways in which things like race and class influence the lives and perspectives of the people in Maycomb. This becomes particularly evident when Tom is accused of sexually assaulting Mayella, he being a black man and she a white women.

Many people in town think that Atticus shouldn't have taken Tom's case, including his sister and several friends. Moreover, several townsfolk would prefer to forego the trial and enact mob justice, despite a lack of proof that Tom is guilty. Indeed, the outcome of the trial suggests that the jury and many others in town have allowed their racial prejudice to influence their willingness to accept the evidence of Tom's innocence.

Throughout the novel, Atticus is constantly being lectured and given advice about how he should drop the case because of the ways in which it's affecting his family; yet he is committed to seeing it through to the end because he is a man of considerable integrity. Atticus believes very strongly in applying the law equally, regardless of things like race or class. In light of that, his willingness to take Tom's case isn't in defense of social justice, but of legal justice. In Atticus' eyes (and the eyes of the law), Tom is as entitled to a fair trial and qualified defense as any white man and he intends to defend him to the best of his abilities.

Given the racial intolerance demonstrated by many other characters, Atticus is something of an outsider. He is reliable because he is willing to do his job to the best of his abilities, regardless of what others think of him. Moreover, he consistently demonstrates integrity by refusing to back down to public pressure and continues to defend Tom because that is what the law demands.