Netherlands/Denmark is an uncommon pairing involving the nations Netherlands and Denmark. It is sometimes referred to NedDen taken from the dutch name, "Nederland" and Denmark.
The two characters had barely any interaction in canon, so most is fandom. 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 fandom, 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, and ect. The nations also have many similarities.
Sometimes, it is said they are attracted to each other because of their similar wild hair. This is also a said reason why some ship this pairing.