Samantha 'Sam' Whittaker is an agent with the British CTA.
That stands for the Counter Terrorism Agency, a very new, elite group within British Intelligence "with a brief to operate outside the niceties of International law; targeting - then eliminating all enemies. The CTA are ruthless hunters in a world where the threat of terrorism has escalated onto the streets."
The CTA sounds like a harsh but unfortunately increasingly needed organization considering the number of home-grown terrorist acts that have been plaguing the nation of late though as we quickly learn following Whittaker's missions, she has become "concerned with the rise of Government intervention within the agency when targets selected for elimination appear to be politically motivated". The concept of "power corrupts" is presenting itself.
Whittaker is one of the CTA's better agent but considering how violent the CTA can get in support of its role, to many people outside the organization, her being so capable means a lot of bad people get hurt very badly, all without the stamp of formal hearings and trials. She has been an operative for some time before we meet how. How many is not certain but her partner in the first adventure mentions they had been together as a team for over five years so that shows us she is by no means a newbie to her work.
Highly dedicated to her mission though not particularly to many of the upper echelon who would rather play politics than get the real job done, Whittaker proves from our first meet that she will do whatever it takes to get close to her prey, including, as we watched carefully, perform impressive acrobatics while on the stage of an all-nude dancing facility.