Names are important to the imagery, allegory, and overall themes of "The Diamond as Big as the Ritz." Let's start with the Washingtons, whose last name descends, Percy explains, from George Washington himself. By connecting the Washington family with one of our founding fathers, Fitzgerald draws a parallel between the Washingtons' own family history and the history of the United States. This connection sets up the parable of the expansion of the U.S. into the West.
We've also got the names of places to consider; by naming John's hometown "Hades," Fitzgerald establishes one of the key elements in his religious allegory. (Be sure to check out "Symbols, Imagery, Allegory" for more on this.)
In a story about a nation obsessed with wealth, social status is an important character clue. The Washingtons' excessive wealth is their single most defining characteristic. Meanwhile Unger is by contrast from an affluent but not entirely elite family in the South. "A function that in Hades would be considered elaborate would doubtless be hailed by a Chicago beef-princess as 'perhaps a little tacky,'" explains Fitzgerald (1.3). Because of this social disadvantage, John is somewhat of an outsider, both at St. Midas and later at the Washington estate.
John's family life is presented to us in the beginning of the text and is later contrasted with the Washingtons'. We know that he is close to his family – his departure is a tearful one – and that he gets his values from them, (he later reflects that "the simple piety prevalent in Hades has the earnest worship of and respect for riches as the first article of its creed." We also know that if he deviated from this standard "his parents would have turned away in horror at the blasphemy" (2.12). Similarly, Kismine and Percy get their own values from their father, Braddock, who must have assimilated them from his from his father, Fitz-Norman. John even notes that "Percy and Kismine seemed to have inherited the arrogant attitude in all its harsh magnificence from their father," in that "a chaste and consistent selfishness ran like a pattern through their every idea" (8.2).