Maisie G is an agent with MI6.
Well, she will be, after a bit.
When we first meet her she is working for her adopted mother, Olga Gabrys, a Lithuanian freelance assassin who works mostly for a group they refer to simply as the 'agency' (lower case so no, it is not the CIA). Maisie is an assistant and a helper and a cleaner for Olga and has been trained constantly since she was little to one day be an assassin herself.
"I have broken endless bones, bashed people, and prepared them for the afterlife," she tells us, but she has never killed anyone herself and is quite grateful for that but she is well versed in how to do so in numerous ways, either with weapons like guns or knives or using her bare hands.
Maisie tells us about her history early on in the first recorded adventure. "Apparently born of British parents, they left me for no obvious reason at an orphanage in Lithuania. This is where Olga first found me when I was five." That is when the training began, small at first with self-defense being the first primary phase and then into other areas needed for Olga's vocation such as spotter and gofer, not to mention learning the best ways to dress wounds because while Olga is very good at her work, sometimes the other guy is as well and Olga is often in need of patching.
The relationship between the two is very interesting. "I provide the money for our lifestyle; you are my sweep, watcher of my back", Olga reminds Maisie. There is considerable love there and Maisie refers to Olga as 'Mom' because now in her mid-teens, Maisie has never known any other parental unit.
This will change dramatically when they are attacked by two assassins for reasons neither of them know and nearly killed and forced to go on the lam to a safe house in England and then ... well, at that point Maisie will quickly (but still anguishingly slowly at times) the truth about her being abandoned and who and what Olga really is and ... her life will changed completely.
Oh, and then there is the matter of Anton. He is a computer AI who runs the safe house that they flee to. And he knows far too well the truth about Maisie and that is major-league exciting and just a tiny bit creepy considering he has 'eyes' everywhere and can talk to her at any time. Without him, though, things that needed to be learned might not have been.