I'm afraid your question has not been answered because it is actually two questions, one of which is kind of a puzzler. I have chosen to tackle the easier question of how a minor character shapes a major character in J. D. Salinger's The Catcher in the Rye. (Frankly, I can't think of any way in which a really minor character shapes a major character in Macbeth. Lady Macbeth certainly shapes her husband, but she can hardly be considered a minor character.) As far as The Catcher in the Rye is concerned, it is pretty obvious that Holden's little sister Phoebe has a big influence on him--even before he goes to see her and is just thinking about her. He realizes that her love and devotion are of great importance to him, and it is she who causes him to decide to come back to his home and family. She also puts a question to him which make his stop and think about the future.
"Stop swearing. All right, name something else. Name something you'd like to be. Like a scientist. Or a lawyer or something."
This is where Holden finally gives the little girl his extremely touching answer: that the only thing he would like to be is a catcher in the rye. But he will continue to think about her question because he loves her so much that he would do almost anything to make her happy--or less unhappy. He may not be able to understand how much unhappiness he is causing his parents, but he can see it plainly in this little girl who adores him.
Phoebe truly is a minor character because she doesn't appear until the very end of the novel. Also she is a minor character because she really is a minor. She is just a little girl who is unable to give Holden any practical advice about what to do with his life because she knows nothing about the outside world (which has been portrayed thus far as a pretty sordid place). All she can give Holden is her love and the entire eight dollars and eighty-five cents she has saved from her Christmas dough.