There is a place -- known only as the 'School' -- where men and women are trained in the arts that benefit those who ultimately become their handlers. Taken from their families as children (though in truth, they are often sold to the School by families in need), these men and women know nothing but their dedication, service, and life for the Regency. While one's education may vary (for the needs of the aristocracy are great), there is a core in service.
Sian does not know from where she came. Nor does she care. She is a tool of the Regency. Trained in infiltration and combative tactics, she was crafted to serve at the side of a lord or general: providing for their day-to-day needs and seeing to their safety. With Cato's recent promotion, she was a gift from his family into his service.
To the outside world, however, Sian is just a newly hired assistant to the Regency officer. The person who manages his schedule and can even sometimes speak for him.