I think you could argue that Boo is the "best friend" in this story. He is always there, no matter what. He leaves gifts to the children as his show of friendship, takes care of Jem when he loses his pants, and, ultimately, saves their lives in the end. To me, a best friend is someone who is always there for you, who "watches your back" and is willing to risk of himself or herself in order to help you out. Boo does this when he emerges from the shadows to save Scout and Jem. Up until this point, he has stayed in the darkness because he is more comfortable there. Scout realizes this, and this is why she takes him out of the light of the house and onto the porch where the light is dimmer. This is also why she takes his hand and walks him home. In true best friend fashion, she returns the favor that he has done by saving her life in the best way that she can. He has protected her and Jem, now she will protect him on the walk home. This is what a best friend does. They give of themselves unselfishly, never asking for anything in return, never expecting anything in return.
I don't fully understand your question, but two of To Kill a Mockingbird's characters who immediately come to mind as friends are Atticus and Miss Maudie. Atticus seems to be the friend of nearly everyone in Maycomb (aside from Bob and Mayella Ewell); even his children call him by his first name and consider him as both friend and father. Many of the townspeople turn to Atticus when they are in need, and he serves as the local representative to the state legislature. Maudie Atkinson is close to all of the Finches--Atticus, Jack and Alexaandra--and Scout calls her "a friend" as well. Maudie is also close to most of the people of the neighborhood.