The following post will examine hints of romance and/or useful
tools that can be incorporated when characterizing this pairing.

Backdrop:

Briefly, it’s important to situate the course of their
relationship.

When they were younger, Hungary used to beat up Austria. Remember
that Austria was originally born as a fighting nation and only became an
indoors person after he resorted to marriage to solve his problems [x]. 

This power-balance later shifts when Austria acts as the
head of the Holy Roman Empire household and Hungary becomes his subordinate [x]. 

Although it’s not depicted, the two are later married under
their dual monarchy. Hungary’s cited birthday (June 8^th) is also
their wedding day and corresponds with the Austro-Hungarian Compromise of 1867 [x]. 

Negotiating Behind
the Scenes:

It would be inaccurate to say that Hungary passively
accepted being under Austria’s rule with open arms, not to mention that it
would disparage the strength of her character. It’s a lot more “complicated”
than that.

Hungary is described as having “defiant feelings” over having been under Austria’s rule. In tandem, she’s also said to have “complicated,
yearning-like feelings towards him,” [x]. 

How I’ve interpreted this before is that the nations have
two selves: the personal and the political. While Hungary may have resented
Austrian power, she didn’t represent Austria the person despite the fact that
he was the embodied personification of the nation in question. Hungary was
ultimately caught between a rut of representing her nation’s best interest and
her own experience as a person in her own right.

Still, even when she was a servant to him, Hungary still
considered those in the HRE household to be her family. Regardless of the difficult
nuances of their political statuses, there were deep personal bonds present that
shouldn’t be overlooked.

Likewise, we also know that Hungary developed a complex over
settling down and feared that her battle skills would get rusty [x]. 

This is far from the case.

Ex: Hungary was keen to fight for Austria during the War of Austrian Succession [x]. 

Aftermath:

As mentioned, the series doesn’t cover the course of their marriage
and instead depicts the aftermath. On the other hand, several hints of romance
are enough to suggest the likely possibility that the two did and still do hold romantic feelings for each other.

Ex: Hungary is too shy to visit the then annexed Austria and
instead asks Germany how he’s doing. Her reason for not wanting to see Austria
is that the state of her economy had caused her to develop a dishevelled appearance.

Either way, it seems that she still cared about how she
looked around Austria. Notice how she blushes too.

Ex: In the same time period, Hungary secretly cleans up
Austria’s room without him noticing.

The most well-known example is during Buon San Valentino.

Ex: Hungary receives a Valentine’s Day gift from an
anonymous source. The deliverer gives a description that matches Austria’s [x]. 

However, it’s not so anonymous to the reader given that the
strip’s title is “Hides his name but not his Mariazell.”

The two are also very close in modern times.

Ex: They spend the Christmas 2007 event together as a
couple. Their relationship is enough to make Prussia—who was already
feeling lonely—cry [x]. 

Ex: They coordinate their Halloween Costumes together—
Hungary dresses up as Austria’s prince in the Halloween 2011 event [x]. 

In sum, while historical tensions may have caused reciprocal
tensions in their political relationship, this doesn’t necessarily mean that
the same tensions were reflected as strongly in their personal relationship.
The two genuinely do care for each other and haven’t seemed to have abandoned
their husband and wife roles; this is on top of the suggested romance between them.