The big problem with BS12 is: what adult and sane person would accept money from some shady orginazation to steal something or kill someone and potentially ruin their entire life and job and make a bigass and powerful company hunt them down. Or is every NT employee a hitman? Lings at least make sense in that regard, as they are hitmen and they got on board by absorbing a guy.
It makes sense when you read and think about some of the Syndicate fluff on /tg/, although it's absent from BS12 fluff page (due to some silliness in it). And the fact that Syndicate is, in fact, a catch-all term for a group of loosely allied groups antagonistic to NT.
NanoTrasen is THE corporation, since they own some very important technologies (like the ability to create psionics). Due to their size, they pretty much can do whatever they damn please, the more unacceptable things they do shoved into off-planet research stations such as SS13.
Doing what they damn please and the rapid expansions, however, mean a number of nasty things. First of all, NT can provide resources using less manpower than small companies (e.g. they can readily make mining mechs and borgs, that drastically improve the time it takes to mine, they can generate a whole lot of energy using solars and singulo that may be selled off and so on). Where NT expands, minor entreprizes die off, their employees now having to find a nice cozy place under a bridge or go and die.
NanoTrasen also has access to really damn good firepower, and slightly inaccurate, but cutting-edge teleportation technology. All that with pretty much no way for planetary governments to make sure they won't, I dunno, invade them? That aside, the technologies are really nifty and potentially profitable if reverse-engineered.
There are rivals - although NT is HUGE, there are still other corporations, willing to raise their position by a little assasination here, a little industrial espionage there.
Then there are ideologically-motivated forces, such as Spess PETA/ALF, opposing xeno experimentation, Spess Anarchists are quite likely a possibility, various kooky religions having their own reasons to hate NT.
AND there are mercenaries for any of those, who might be on the run anyway for their own reasons.
So, to recount: the station might be infiltrated by people screwed over by NT expansion, various black ops, corporate commandos (alliteration, yay) and terrorists, pick as many of those as you wish.
Remember that in the grim darkness of the future, there are Genetics and Plastic Surgeons, both of which can make you literally untraceable once you can evade capture.
), and that lends itself to more eccentric characters.