Geraldine 'Gerry' Tate is an agent with MI6.
We first encounter her when she is waking up from an incident that left her briefly unconscious in a crashed plane in the ocean, both pilots of the craft dead from being shot by her before the plane hit the water and started to sink. Most of her activities that we can follow subsequent to that are not quite as harrowing but some do get fairly close.
In a flashback just over a decade before in 2003, we watch Tate in another interesting situation where she was in Kuwait went by MI6 to vet a British diplomat suspected of selling secrets to Russia; she finds proof it was the man's girlfriend who was really an undercover FSB operative but that discovery launches more gunplay and both girlfriend and diplomat end up dead. Tate was having a bad day (though they were having a worse one).
The two incidents might leave a person with the feeling that Tate is particularly gun-happy or disaster-prone but in truth the woman is darned good at her job but she does tend to respond to violence against her with equal and opposite force - though maybe not so equal since in both of those cases the opponents died and she did not.
It is important, however, to note that part of her job with MI6 was in fact to, on occasion and when directed, terminate with extreme prejudice as the Cousins would say and she was not squeamish about carrying out those orders. But when a mission goes wrong and she is made the scapegoat and finds herself under arrest for murder, she is not hesitant to voice her displeasure. That will not change anything, though, and shortly after we meet her, she will be convicted of murder and sentenced to life in prison.
Most of the subsequent activity we follow Tate in comes after MI6 approaches her some time later with an offer to get her released from prison with a cleaned record and a chance for a new life - if she will do just one more thing for them.
Understandably, she agrees. Even more understandably, she does so with a very long memory and a lot of resentment.