Mrs. Dubose is the meanest old lady ever.

Scout describes Mrs. Dubose as “plain hell” because she picks on the children. 

Mrs. Dubose lived two doors up the street from us; neighborhood opinion was unanimous that Mrs. Dubose was the meanest old woman who ever lived. (ch 4)

Mrs. Dubose is described as “close on to a hundred” (ch 10), and she likes to insult people as they pass by.  Atticus is always kind to her, and tells her she looks like a “picture.”  He also tells his children to be polite.

When Jem loses his head and ruins Mrs. Dubose’s flowers because she insults Atticus, Atticus makes him go and read to her.  He tells Jem later that Mrs. Dubose needs the reading because she is trying to wean herself off of morphine so she can die on her own terms.  Atticus wants Jem to see what real courage looks like.