Denmark/Sweden is a less popular slash pairing involving the characters of Denmark and Sweden. The pairing surged into popularity alongside Norway/Iceland back when Denmark and Norway's designs first came out and the western Hetalia fandom was growing rapidly. The pairing seems to have dropped in popularity over time while Denmark/Norway has grown and Norway/Iceland has remained popular. The Denmark/Sweden pairing is often referred to as "DenSu", "DenSwe", "SweDen", "SuDen", or "SweDenmark".
Sweden and Finland are stated to be running away from Denmark's house during the "Running Away with Su-san" strip because they are tired of living with the loud, bossy Denmark who doesn't listen to anyone.
The two both appear but aren't interacting directly in the "The Nice Products that Come From Scandinavia Are...", and "Nordic Lines" strips. When Denmark mentions looking for the other Nordics during the Hetalia Fantasia 2 Drama CD, he struggles for a moment on Sweden's name.
The portrayals are largely the intense, violent love/hate variety due to Denmark being depicted as mentally unstable, emotionally and physically abusing others, and seeking dominance over a resistant Sweden. A lot of these portrayals may come from the fact Denmark and Sweden fought against each other the most - and it was usually initiated by Denmark. Sweden might or might not have sexual and/or emotional attraction back, or might be portrayed struggling with feelings faintly like Stockholm Syndrome. The opposite is much rarer, where Sweden takes revenge on Denmark, or even excessively abuses Denmark, whether the Dane is depicted as twisted and abusive in the past or not. Sweet and healthy moments can also be portrayed, particularly during modern times from the closeness of the modern Nordic countries and the Øresund bridge that connects the countries in real life.
Denmark's eventual depictions in the comics doesn't match up with this extreme and Himaruya's scarcely touched on Denmark and Sweden's history, beyond a mention of wanting to get away frm the bossy Dane in the "Running Away with Su-san" strip. The fannon depictions come from the historical struggles during the Kalmar Union, the Stockholm Bloodbath, and the next two centuries of intense rivalry and wars during Denmark's fall from and Sweden's rise to power in the Baltic. Again, more stable and affectionate portrayals can come from the countries' comeraderie since the mid-1800s, as well as a general wish for a happier couple.