When Francis talks to Scout he reveals an unpleasant feature of Aunt Alexandra. What is this?
I assume that you are talking about what Francis says to Scout in Chapter 9 when the clan is all at Finch's Landing. The things Francis says to Scout while he is making fun of her make it quite clear that his grandmother (Scout's Aunt Alexandra) is racist and also very arrogant and stuck up.
Francis tells Scout about Alexandra's attitude towards Dill. She clearly looks down on Dill because of his family situation. She compares him to a stray dog. Aunt Alexandra has also, apparently, been saying what they thought was bad things about her brother Atticus. She has told Francis that Atticus is a "nigger-lover."
By saying those things to Francis, she has shown that she thinks she is better than others because of her family background and she has shown that she is racist.
She is found out to be a racist woman and this is understood when Francis explains that she looks at Dill like a stray dog and she also believes she is high and mightier than people who are of color. This is also demonstrated when she tells Atticus to let Calpurnia leave but he would not have this.