They owe their
existence to them. 

Ex: France lays out the rule that as long as the nation
possesses a stable government and a large enough citizenry to identify with
said national identity, the respective personification will remain and continue
to be immortal.

This rule is further confirmed in World Stars, where France
explicitly admits that he exists because of his citizens.

They openly interact with one another + humans know of their existence. 

A common misconception that people often make is that the
nations’ identities are a secret kept from the public and that only their bosses
know about them. That simply isn’t true.

In fact, while some humans are shown to be perplexed by the
nations’ immortality, their overall existence is known to the public.

Ex: France introduces himself to Lisa, Jeanne d’Arc’s
reincarnate.

Ex: Italy flirts with his own citizens.

Interestingly, Italy’s also admitted to getting tired of
seeing the same faces of his citizens every day – a consequence of his
immortality. However, he does add to this and claims that seeing
these same faces can also be refreshing.

Point is, on top of their being no knowledge barrier of
their existence, the personifications openly interact with their citizens.

The Nations are Citizens Themselves. 

Ex: Romano has his own passport.

Ex: Russia isn’t exempt from civilian duties, such as manual labor.

He’s forced to build a waterway alongside his people under
impoverished conditions.

He fought alongside his people in both world wars (as well
as any other personification), and endured the same notoriously terrible living
conditions that his soldiers were exposed to.

Ex: Coinciding with the last example, the nations serve in
their military. Here, America is tasked with tightening security among his
troops after Germany and Prussia successfully infiltrated them.

Population Demographics Affect the Nation’s Personality Traits, Behaviours, Habits, and Physiology:

Given that this is a series predicated on stereotypes, this
isn’t all that surprising.

Ex: China has a video game addiction resulting from this
becoming a rising problem among Chinese youth.

Ex: Germany strictly adheres to the directions of his GPS.
Hima drew this from a news story of a Bavarian who caused an accident by
following GPS directions.

Ex: Post WW1, several Lithuanians immigrated to America to
outsource their labor so that they could send money back home. As such,
Lithuania lived with America during this time.

The same thing happened with Romano. He went to live with America
during the first Industrial Revolution –a direct effect of his own people
migrating there for work.

Ex: As a result of his shifting cuisine (i.e. Indian and
Chinese influences), England is beginning to learn how to cook.

Ex: A nation’s obesity rate likewise impacts the weight of
the personification.