Raised as part of a nomadic band of halflings traders, but found it too stifling/not exciting enough (they stuck to main roads/cities only, and were very cautious/unadventurous), and slipped away into a nearby big city as a young teen.
Forced to fend for himself, he taught himself to pickpocket/steal/etc… to make a living, however his morals meant he would only steal from those who could afford to lose it.
Tried to pickpocket a gentleman, only to have his brother, the captain of the cities watch catch him and put him in jail, however the gentleman (while being an upstanding member of the community, curator of a local art gallery) had some shady connections and saw an opportunity. He defended Sanmo to his brother, claiming he was afraid of pickpockets, and thus had hired Sanmo to train him to defend from pickpockets through practice.
The captain of the cities watch is Sanmo’s adversary, never trusting Sanmo after catching him in the act, even with his brothers defense, and tries to catch Sanmo at every turn and watches him carefully when hes in town.
The art curator with criminal connections is Sanmo’s benefactor, not only preventing his brother from throwing him into jail, but also offering him leads on good joints to steal from in exchange for a cut of the profits. In exchange, Sanmo keeps an eye out for especially unique art pieces to return to the gentleman for his own private gallery.
Sanmo has always lusted for a challenge and excitement (thats why he left his family in the first place), and so he can’t resist stealing something if it’s put to him as a challenge.