Dill and Scout actually have a few things in common, including things they "lack." This is part of what makes them such close friends.
Parents: Both children lack a relationship with one of their parents. For Dill it is his biological father and for Scout it is her mother. Dill's father has been out of the picture (although alive) for the majority of his life, and Scout's mother died when she was just a baby.
Traditional Family: Because of the lack of a two-parent family, Dill and Scout both lack the traditional family that was so common in that era--two parents and their children all living "happily" together. Dill spends summers in Macomb with relatives. Scout lives with her widower father and older brother, and in spite of societal expectations, her father has not re-married. In many ways, Calpurnia acts as a stand-in mother for both Scout and Jem.
Traditional Gender Roles: Dill is described as quite small for his age and somewhat effeminate. As a result, he does not fit in with other boy groups his age. He is surrounded only by female relatives, so most of his influences come from women. Scout is his opposite in some ways--she is a tomboy who does everything she can to avoid being considered ladylike. She lives in a home with her father and older brother, and so her lack of a strong "ladylike" presence in her life has contributed to her tomboy ways.