In Chapter 9, which character supports Jem and Scout?
In this chapter you could make the argument that Uncle Jack and Atticus are both supportive.
Uncle Jack: Scout has a run in with Aunt Alexandra's grandson, Francis. He says that Atticus is a "nigger-lover" (the book's wording, not my own), and Scout retaliates, in typical Scout fashion, by physically attacking him. At first Uncle Jack reprimands her, but after they get back home and he hears her side of the story he says that he would have liked to have hit Francis himself. He bandages Scout's hand up for her. He later talks with Atticus about the tension in the case and how the kids might face some challenges as a result. This conversation is purposefully loud enough for Scout to hear.
Atticus: Atticus explains that he has taken a difficult case, and not one he especially wanted. That being said, he's going to do the best job that he can. He also explains that Scout and Jem might hear some things about him they don't want to hear and if so, they need to just ask him about it.