Ali Sinclair is an agent with MI-6.
Was an agent, to be technically correct. Was up until she was caught during an assignment in Mexico and thrown into a women's prison on the outskirts of Juarez. Abandoned by her bosses like she expected. Left to live her next seven years in that very unpleasant location and already having existed there for the past two.
The legal records south of the border indicate she is a drug smuggler. The legal records at Vauxhall Court, if you could actually see them, might show that she is (was) a "talented undercover operative" for MI-6, able to adopt a role and stick with it no matter who she had to lie to or shoot at.
Sinclair is, when we first meet her, 31 years old. How many years she was an agent is not known but it had to have been more than a couple to get the reputation she had but it could not have been too many because prior to joining the Secret Intelligence Service she had been a captain in the British military, Intelligence Corps, Special Reconnaissance Regiment. She had "frontline tours of duty in Iraq and Afghanistan". This was a still relatively young woman who had more than paid her dues.
A major part of those dues is pain. The ability to inflict it quickly and thoroughly and most decisively, something that Sinclair can and does do without hesitation. And the ability to take it when it comes because, no matter how good someone is, and Sinclair is very good at what she does, someone or someones are going to resent it and the pain will come fast, hard, and often.
Having her back, and she his more than once, is Frank McGill. She and McGill had become close friends while in the Middle East and later working at MI-6 their deep bond had just grown stronger. While there was no physical intimacy between the two, the emotional intimacy was unbreakable.