Netherlands/Denmark is an uncommon pairing involving the characters Netherlands and Denmark. It is sometimes referred to NedDen taken from the Dutch name for Netherlands, "Nederlanden" and the English Denmark.
The pairing is a fandom-based pairing due to the lack of interaction between the two characters. In the 2011 Christmas event, it was shown that Netherlands beat Denmark up when Denmark went to befriend Japan. Because of this, the pairing is seen as a love/hate pairing at times.
In most NedDen fanfictions, it is said that Netherlands is one of Denmark's best friends next to the Nordics. The reason for this is because the two countries in real life have a close relationship as allies and trade partners.
Sometimes, it is said they are attracted to each other because of their similar spiked hair. This is also a said reason why some people like or ship this pairing.