The relationship between all these companies is as follows based on my understanding:
-NCSoft in Korea is the overall owner of BNS. They're the primary developer, they create virtually all of the content and features, and are the final say as far as Blade and Soul goes for it worldwide.
-NCWest is their NA and EU branch. They used to have a separate branch just for EU, but they were merged into NCWest before we got BNS. They manage the NA and EU servers and apply updates and support to those servers, adding little to no content of their own other then translation to other languages. They are completely owned by NCSoft however, and answer directly to them. Supposedly they used to have a lot more say, but now they seem to only apply patches mindlessly to the game, sparking all these threads about broken things.
-Tencent operates the Chinese server. Unlike NCWest, they do have some say over content they get, and have a few unique features, though it is unclear if they developed them personally, or requested NCSoft to produce the patches for them. They do not seem to have a stake in NCSoft, or if they do, it's a relatively small stake without much control in the matter.
-Another company operates the Russian servers, though I'm not sure of their name. Much like Tencent, they operate that server and that server alone, but do not seem to have any unique features, though we have seen cost differences for things in the past for upgrades and events suggesting they do have a bit more say then NCWest does.
-Netmarble (not mentioned yet, but worth it for the sake of completion) operates BNS Revolution for mobile devices. They actually DO have a stake in NCSoft, about 8-9%, but have not used this to influence BNS on PC as far as anyone can tell.
I hope this clears it up and resolves the debates in this topic.