Scout and Jem, for most of the novel, don't really like Aunt Alexandra. Their Aunt comes from the old southern school of etiquette. She does not approve of how Atticus is raising his children. She feels that they have far too much freedom to play and state their opinions. Aunt Alexandra feels that Scout, in particular, does not act like a proper young lady.
Jack is fatherly, but Scout feels she can confide in him; he
keeps her secrets and she trusts him.
