Swamped

Stinger is probably the best choice. He's a cast-away that drifted here from a distant land, and one of the most stealthy individuals in the marshguards.
He doesn't talk much, mainly due to his stilted understanding of the language, but what you do understand about him is that he came to the marshguards after having a rough go of it in the surrounding kingdoms, struggling to find a place in the cities and towns. He says he much prefers the swamp as it reminds him of his home, and the marshguards remind him of his culture. Trained in the art of guile and quiet surveillance, you think he might have been an assassin in his old country, but that's just your best guess.
You'd be an idiot not to bring him.