Marie Declerq is an agent with the CIA.
Nadia Kuznetsov is a freelance Russian assassin.
They are the same person. She who becomes known as Declerq spent most of her life as Kuznetsov. A third of the way through the first recorded adventure we have of her activities, we are given a dossier on her compiled by the CIA and read by a DCI Madison to his bosses on the lookout for a suspect in a horrific terrorist bombing:
"Born Nadia Kuznetsov in Moscow, Russia on 16 February 1985. No detailed physical description available, as this was scrubbed from the electronic file. Both parents ... were former FSB agents, who were killed in a covert operation that went bad in 2001. Prior to that, they worked for the KGB during the second half of the Cold War.
"Highly proficient in hand to hand combat, a very high calibre markswoman, and a highly accomplished intelligence analyst; all courtesy of her time in GRU Spetsnaz ... Initially invited for an assessment by the FSB at the age of 16 in recognition of the service her parents had provided to their country and formerly to the USSR. It was a calculated risk, based on her genetic profile and upbringing. Fast tracked into the GRU, and then into GRU Spetsnaz, based on outstanding test results, and sponsorship from a high ranking FSB official, Vladimir Kuznetsov, also her uncle. Considered to be one of their finest ever operatives, but attitude issues made her a difficult asset to manage. Consequently, had a number of blemishes on her service record for challenging commands that she considered to be beneath her or ill-conceived. Dishonourably discharged in 2008 after leaving a more senior officer badly brain-damaged, after he had allegedly attempted to rape her.
"Freelance assassin since 2008. Formidable market reputation - has never failed to deliver. Consequently commands a premium over market rates. No known religious or political ideologies or affiliations. Currently goes by the moniker of 'death dealer'."
While no stranger to fake identities, Kuznetsov will take the persona of Marie Declerq for that first adventure and when she is subsequently offered a job with the Agency as a result of her actions in that adventure, she sticks with the name. Her employment with the Agency will come after her clients will prove to be as much a danger to her as she was to those they hired her to kill. Those who betray her find, as those clients did, that she is not a forgiving kind and forgetting is not going to happen.