Belgium has made only three appearances so far, and two were cases where he was invited to a party where many countries gathered. Since the nation of Belgium is known for chocolate and comic books, he is depicted as a chocolate-eating nerd with bristly brown hair, opaque round glasses, and freckles/pimples. However, he can drink with the best of them, referring to Belgium's beer culture and its large breweries.
Not much else is known about him, though he seems to be friendly with the Netherlands. Sister Japan thinks the two of them would make a cute couple. The Netherlands doesn't like the idea of being paired up with a geek, but Belgium himself doesn't seem to object to the idea in principle.