Although the geography of Japan suggests a north-south orientation, its political geography over the past 1500 years has been defined east and west by the tensions between Kyoto and Tokyo.
In 794, Emperor Kammu moved the ancient capital twenty miles north from Nara to Kyoto. In the wake of the Genpei War (1180–1185), Minamoto Yoritomo shifted the seat of power to Kamakura, thirty miles south of Tokyo. The Ashikaga shoguns returned it to Kyoto in 1336.
Tokugawa Ieyasu established his government in Edo (Tokyo) in 1603. With the overthrow of the Tokugawa shogunate in 1868, the capital of Japan was permanently relocated from Kyoto to Tokyo.
These two regions are now known as the Kanto ("eastern gate") and Kansai ("western gate"). The Kanto is home to Tokyo and Yokohama, the first and second largest cities. The Kansai is home to Osaka, Kobe, and Kyoto, the third, sixth and seventh largest cities.
The novel mostly takes place in Osaka's Tennoji and Sumiyoshi wards.
During the Edo Period (1603-1868), the Tokugawa shoguns imposed an authoritarian form of federalism on the provinces. This was to keep discontented governors from banding together and overthrowing the regime (exactly what happened in 1868).
Traveling from one province to another required an internal passport. Getting caught without one would get you tossed in jail (though the draw of the big city was enough that many risked it). As a result, the provinces developed distinct identities and dialects.
As the gateway to the more "refined" Kyoto, Osaka was the center of commerce in medieval Japan. The traditional Osaka greeting is "How's business?" Properly dubbed, Osakans should sound like they're from New Jersey. "It ain't personal, it's business."
The Kansai region shares another similarity (or invidious stereotype) with New Jersey. It is home to Japan's largest yakuza syndicate, the Yamaguchi clan, along with several smaller crime families.