The Shimon family, as a sign of their re-connection and new-found alliance with the Vongola, make their home in Japan, keeping watch of the Asian continent and homeland of two of the greater 'Bosses' of the Vongola. Japan seems to be flooded with a lot of mayhem thanks to the Shimon family's eccentricity, and 'rival' families seek to topple their control.
Excluding Russia protruding in, China is practically the largest country in size and population on this continent. It also happens to have a government propped up by the Shimon, and will funnel the massive amount of resources the nation possesses to the family. As a matter of fact, the Shimon is the second most-dangerous family in the world[The Millefiore is first], since they have direct access to nuclear weapons of mass destruction by default via their control over the Chinese government.
Still seperated from its Northern counterpart, South Korea acts as a backup or fallout shelter for the Shimon in case of emergency. It's also noteworthy that the ONLY reason why the Northern half has been incapable of being able to make substantial effort to merge with this country is due to the Shimon's intervention. Attempts made by North Korea are sabotaged or deterred by the Family's presence.