Natalia Nicolaeva is a vigilante.
I started to describe her as a survivor since considering the terrible things that happen to her when she had the audacity to try to make a better life for herself than chopping grain with a scythe on a tiny impoverished farm in her native Moldova. That last is where we meet her at the beginning of the first recorded adventure. She might have still been living there with no real prospects other than to marry someone she probably would not truly love and work another tiny farm with him, had it not been for a friend who was told of a great opportunity to work as a waitress in distant Italy. The friend needed someone to go with her for safety and reluctantly, Nicolaeva agreed. That will be when needing to be a survivor steps in.
But I then almost put down as her 'occupation' that of a student because that is what she will become after a while and it is as such that she will be first approached by MI5 and asked to help in a matter. 'Ask' is putting it far too politely but then Nicolaeva has gotten pushed into things in a lot rougher ways and has managed to survive.
Nicolaeva has her stories told in this compendium because in trying to first survive and later do the right thing, she will come up against international criminals, Russian computer hackers, corrupt government officials, and even hired killers. And after holding her own against those dangers, she will work on an ad hoc basis with both British Intelligence, as mentioned above, and then with a CIA operative.
Back to the 'vigilante' moniker assigned her - it is given because there are several times that she could have, once out of immediate danger, left it behind and sought safety. That is not, though, the kind of person that she is. As mentioned in one particularly nasty situation, she does "whatever she can to make the killers pay for their crimes". She might not have formal training in such matters but what she 'can' is quite a bit.