Isabella Rose is an agent with Group Fifteen.
Well, she is an unofficial one because a highly covert department in British Intelligence, responsible for eliminating with extreme prejudice enemies of the country, could not be known to employ a 15-year old girl. It should not be known to employ anyone, of course, since it did not exist on the books but had it been publicly talked about it most definitely would not want people to say it used young girls to spy and possibly kill for it.
Not that Isabella would have worried about such things. By no means psychotic or blood-thirsty, Isabella nonetheless is well used to the life of a Group Fifteen agent. She learned how to survive and how to stop others from doing the same starting when she was 11 and her absent mother reentered her life. She learned when it became apparent that the very people who had killed her father and stolen her from her mother when she was only 3 would do anything they could to killing the two of them now that they were back together. Luckily for her continued existence, her mother, Beatrix Rose, had once been one of Britain's deadliest assassins and knew ways to stay alive and to kill others that few would have imagined.
When the younger Rose was barely out of infancy, her mother had learned on a mission facts that her crooked boss had not wanted known. He decided the chance she might reveal something was too great so he ordered several of his subordinates, those who would kill without ever asking questions if the price was high enough, to visit the mother at home. It was there that the agents shot her father in the head, severely wounded her mother and would have been killed herself had not a truce been hastily reached. In exchange for her daughter's life, the elder Rose would disappear.
"Control", as the leader of Group Fifteen was known, secreted her away in a "series of foster homes and institutions" until another former agent with his own grudge, , stepped in and returned her to her grandparents and later to her mother. The foster years were less than pleasant for the young Rose and not something she enjoyed remembering or discussing.
Once she was reunited with her mother, one sad fact was inescapable - Control and those who had carried out the attack on the Rose family would not rest until Isabella and Beatrix were dead. So while there were many instances of getting to know each other, there were even more sessions of training to get the younger Rose equipped with the skills needed to survive the upcoming war. Hand-to-hand combat, multiple languages, marksmanship, street smarts - these were all various skillsets that her mother trained her in during the brief couple of years they had together while the elder hunted Control and his former minions. Rose's mother, wracked with pain from rapidly spreading cancer, tried in the end to destroy her former boss but came up short. Isabella Rose stepped in to finish the job.
Some time later, the new Control, a man far less evil than the previous leader and a man who helped the Rose's when they needed a friend, needed someone young with a particular ability at survival to take down a very nasty group. He thought of Isabella Rose. She said yes.