Netherlands/Belgium is an uncommon pairing involving the characters Netherlands and Belgium. It is also known as NedBel, deprived from the first three letters for the Dutch version: Nederlanden and the last three letters of Belgium, also called NethBel or Tulip Silblings.
Netherlands and Belgium are siblings alongside Luxembourg. Netherlands is known as the older brother meanwhile Belgium is the younger sister. Their relations as children have been elaborated on in Episode 109, in which Belgium brings her brother to her house, lavish and rich compared to his humble home. It is because of this that she admits that Netherlands' cheap manner may have partially been her fault. Netherlands and Belgium are said to have had difficult relations, stemming from when they were both under Spain's control. When Netherlands left to become independent, he fought against both her and Spain in the Dutch Revolt and the Eighty Years' War.
He later tried to reconcile with Belgium so they could both form the Kingdom of the Netherlands, but due to their irreconcilable differences (Netherlands being better with trade and influenced by the Protestant British versus Belgium being better with farming and influenced by the Catholic French), it did not work out and Belgium declared her independence.
It is said that Belgium often gets pushed around by Netherlands in various activities, even though it's not shown in any appearances. With Netherlands being a clean freak and very indulged in money-making, Belgium's one of the few countries that could keep him in control as she's the younger sister that's very concern in his brothers' actions.