This is an interesting question, which is complicated. First of all was the situation in the northern and southern Low Countries not that different when it comes to identity in the middle ages, this split in identity only really started in the seventeenth and eighteenth centuries.

In the middle ages and the early modern period, people in the Low Countries would primeraly define themselves as the resident of the city or village they lived in. The state didn't participate in well being and care, its main participation in the lives of it's citizens were collecting taxes and military protection. The city would sometimes have a hand in other aspects of your personal life. For most people their lives were determined by the people whom they lived in proximity with. Your professional life took place in the guilds, or was protected by it through your connection with a member of a guild. Your social protection was more guaranteed by inhabitants of the same city, the guilds or church. A person didn't have a state nationality, you were a "poorter", "burger" or citizen of a city which granted you some rights and duties. These city rights were especially important in protecting themselves against the duke or count of your "dominion" which the lord needed to affirm when inheriting the title through "Blijde inkomsten" in each important city.

This doesn't mean that there wasn't a sense of affiliation with the larger duchy or county someone lived in. There would be a connection with the dominion you lived in. A Brabantian would through his/her connection with their lord know they were Brabantians, the same for Flemish and Hainaut people. Some historians do say that even a bigger sense of connection would arise under the Burgundian dynasty from the fourteenth century onwards and certainly under Philipe the Good who set up a administrative system that connected all (or most) of his dominions. Most administrative personnel that would be present in a dominion would be from another dominion, this was the case for all his dominions (f.ex. Holland, Flanders, Hainaut) except for Brabant. He also introduced a single currency, de Vierlander, and set up a States General which would function as an intermediary between the lord and all his dominions in the Low Countries. Some of these efforts for unifications where kept, some fell out of favour. This would plant the seeds of a sense of belonging that according to some made the Dutch Revolt (1568-1648) successful in the Early Modern period. Still, the citizenship was determined by your city or village you lived in.

That being said, the Low Countries where as a collective known under many names in other countries. Due to the trading power of Flanders, the inhabitants of the Low Countries would sometimes be labelled as Flemish. Even people from Brabant or Holland. This is reflected in names of Art History like the medieval Flemish Primitives and the early modern Flemish Masters. It is also known that some inhabitants of the duchy of Brabant didn't like to be called Flemish. On maps the Low Countries in general where often called Germania Inferior or Belgica. People would sometimes define themselves as Netherlandish/Dutch, Belgian or Flemish, but this was to be understood in the broad sense of all the Low Countries, not only the Southern Netherlands. This shared feeling would start under the Burgundian Dukes, would reach its climax at the early stages of the Dutch Revolt, when later in the second half of the eighteenth century the term Belgian would be especially for Belgium and Dutch for the Netherlands. The term Flemish would have fallen out of favour to describe all people in the Southern Low Countries during this time and would rise up in the nineteenth century under Flemish nationalism for the people who live in predominantly Dutch speaking area's.

If you would be able to go up to a medieval person, he or she would probably identify him- or herself by city or heerlijkheid. Small chance on own county/duchy identifier or Flemish or Netherlandish. The idea of a nationality in the sense of identifying to your nation, is an idea that only becomes dominant in the second half of the eighteenth century.

EDIT: a book on the broader subject of citizenship is: "Citizens without nations" by Maarten Prak or in the Dutch version "Stadsburgers" by Maarten Prak.