Uncle Hammer is the brother of David Logon, who is the father of the story's narrator, nine year old Cassie Logan. Hammer and his temper live up north in Chicago, an arrangement that his brother, David, pronounces as being preferable because Hammer's fuse is pretty short when it comes to white people. Interestingly, Hammer drives the same car as one Harlan Granger, who while not necessarily as much of a villain in the story as the hateful Wallace family, is certainly not one sympathetic to the struggles of any African-American. Granger is a local landowner who seems to be living in the past, according to Cassie's grandmother, and is unwilling/unable to let go of the need for white supremacy in the community. If Granger were a woman, he might be described as the "queen bee" of the area; he is the unsanctioned, but de facto "leader" of the racists in the community, although his activities tend to lean more toward intimidation through implied threats, rather than the more overtly menacing methods of the Wallaces, who were known to have killed at least one black man in the area over a perceived slight.
And so, when Uncle Hammer comes to Mississippi for Christmas driving his just-like-Harlan-Granger's-except-it-might-actually-be-newer-and-nicer Packard, the Logans are gleeful as they take a ride around the country roads, and Granger is incensed. To the independent Logans, it's a thumbing of their noses, so to speak, of Granger's insistence on keeping blacks "in their place" in the community, especially since it's a perpetual thorn in Granger's side that the Logans are also independent landowners. To Granger, as with the Logans' land ownership (which used to be part of his family's property, and his wishes to buy it back have been met by the Logans with polite disinterest), Hammer's Packard is another symbol of what Granger fears most, the possibiity, however small, that race relations might relax and blacks in the area might cease to "know their place" as the inferior race.