Dolphus Raymond is a local "drunk"; or at least, that's what everyone thinks. He is a white man from a good family, but he has chosen to live on the black side of town with his mistress, who is also black. There, they raise their children, & live in relative peace. He is a source of rumor & speculation amongst the townspeople, because of his chosen lifestyle.
His secret? Mr. Raymond is not an alcoholic. In fact, all he carries in his brown paper sack is a bottle of Coca-cola. When Dill leaves the courtroom distressed, Mr. Raymond shares the soda with Dill, and talks with the children about tolerance and love. He explains that letting everyone think he's drunk helps them deal with the fact that he's chosen to ignore social standards, and instead pursue happiness with the woman he loves. He understands Dill's sickness at the cruelty he has witnessed during the trial, and explains that when they are grown, they'll no longer cry about the injustice in the world.
Mr. Raymond is another adult, like Miss Maudie and Atticus, who teaches the children a lesson in equality, and exteding compassion to all people. He shows that there are others in Maycomb who stand behind Atticus' decision to represent Tom, whether they can speak up in public or not. Mr. Raymond has chosen to keep a small secret in order to maintain his position in life, & he shares this with the children because he sees that they, unlike the other adults, will understand.
"But why had he entrusted us with his deepest secret?" I asked him why."Because you're children and you can understand it,"