Scandinavia: The Netherlands and Denmark are good friends--not as emotionally close as the latter is with Norway, Sweden, or even Germany, but the two of them enjoy a lot of the same things, including bicycling, teasing Sister Japan and Germany, and probably sex. It has not been revealed what Norway and Sweden think about all this, but they probably don't care and indeed Sweden has no problem inviting The Netherlands to his dinner parties. Freetown Christiania seems to view him as a role model, or at least an excuse.
Family: Sister Netherlands has made a couple of appearances, though never with The Netherlands himself. It's anyone's guess as to whether they get along or not, though it's worth noting that she comes across as much more sweet-natured than he is.
The World: Sister Japan thinks The Netherlands should hook up with Belgium. He doesn't find Belgium attractive at all, however...which may be why he gets such a kick out of teaming up with Denmark to spring "yaoi surprises" on her. Germany is frequently the vehicle by which they do this, although it's uncertain whether he is equally an intended victim or just caught in the crossfire. Based on the behavior of all three at Greece's drinking party, there might even be some genuine mutual attraction.
The Netherlands also seems to be on friendly terms with Romania, at least insofar as they can re-enact "Dracula" together. He's been known to hang out with his former colony South Africa in December, so that they can portray St. Nicholas and Black Pete together--with the Netherlands as Black Pete, because it would just be silly to have him played by an actual black man!
The Netherlands is depicted as a lean young man with poorly combed brown hair. He is always shown wearing jeans, sneakers, and a tee-shirt with the design of the Dutch flag, and frequently has a lit joint on his person.