Nikita is an agent with Section One.
That agency is billed, by themselves, as a clandestine anti-terrorist organization. Early in her training, Nikita is told by her trainer and handler, Michael, "Section One is the most clandestine organization on the planet. It's our job to bring down the criminals and terrorists that no one else can get." To do this, it employs tactics that no official government bureau could ever pretend to use: extortion, manipulation, torture, and murder. The concept was simple; the bad guys were very bad so to stop them, you had to be worse. And they were.
Section One found a very interesting way to staff its needed number of operatives. They trolled the prisons looking for inmates that had already racked up a few victims in their civilian lives. They faked the deaths of those they considered viable and then set about training and indoctrinating them. If a trainee proved too untenable or unwilling to become part of the group, well, they were headed towards a bad end already and the world thought them dead so finishing the job was expedient.
That is where Nikita came in.
She was in prison for the murder of a police officer, sentenced to life without parole. Her suicide was faked and she was whisked off to the training facility to begin her new life as a Section One operative, just another hardened killer being used for the greater good.
The problem was that she had not killed anyone. In an organization filled with killers being taught to become better killers, she was the only one who had never taken a life. Unfortunately, to keep hers she would have to adapt.
Now, do not get the idea that just because she is innocent of the murder she has been sentenced for, Nikita is an innocent waif. She is innocent, of course, of killing someone but from the moment we first meet her, she has been living on the street for years and has more than a little talent at survival. She also has more than a normal share of attitude as shown when she first walks the corridors of Section One; she has no idea of what she had landed in but she seems prepared to challenge it nonetheless.
A couple of point to know about Nikita and her new life.
One is that while she does learn to be almost as hardened as Section One wants her to be, her code of ethics does not die along the way. She was really a good person at the start and even with all the things she has to do in her job, she stays basically a good person/
Two is that for all its talk about defending democracy by doing bad things to bad people for the greater good, Section One has some pretty nasty skeletons in its ever filling closets.