He is described as, "Huge. Not built like a weight lifter or anything, just a naturally big-boned, brawny man standing almost as tall as I. His dark hair was gathered at the nape of his neck with a bit of ribbon. A long, dark blue coat fell to his calves, its sleeves marked with gold braid. Beneath that, he wore a uniform—a tight-fitting blue jacket, white shirt, white pants, and high black boots."
He wields a long-handled axe, a flintlock pistol, and a knife. The pistol uses bullets made out of memories, which can harm vengeful spirits. It's his symbol of authority over the army of ghosts.
He once stated all cats can see ghosts, about one in ten dogs and sincere priests and shaman who care for other's souls 
For three centuries he had been a guardian and felt that he failed in duty when the Grey Ghost abducted Mortimer Lindquist. Uriel told him that he'd watched over Lindquist faithfully, and since he's grown so much in the last few years, he'll be just fine. Stuart's memories are in his bullets which he uses in his fight against evil ghosts.
He's existed as a ghost in Chicago for more than two centuries. He stayed on earth in order to guard his descendants, including Mortimer Lindquist, and teach new spirits and help them move on. As to why he has the title of "Sir", and a British accent, yet fought against the red coats is "a protracted and complex tale."
Guardian spirits of Lindquist's house were more than spirit soldiers. They were "ghosts of duty and obligation unfulfilled, who had remained behind to see their task to completions." They were shades of faith, love and duty, serving as a balancing energy against the dark power of the killer ghosts.
In the seriesEdit
In Ghost Story, Harry Dresden first encountered Sir Stuart on Mortimer Lindquist's porch threatening to shoot him. A wraith tried to get Dresden's ghost and Stuart kills it with a memory bullet. Then takes him in to see Lindquist who declines to help Dresden. He and his forces do battle with six lemurs and their attack wraiths in front of Lindquist's house. The Grey Ghost zapped him with an electrical jolt and Dresden punched it; and Stuart scolds him for it. After a gunman tried to kill Lindquist, Sir Stuart helped convince him to help Dresden. He waited with Dresden watching the meeting of the Chicago Alliance and Paranet. 
Mortals working for the Grey Ghost abducted Lindquist and burned his house down. Dresden found Sir Stuart trapped in a circle of fire made with gasoline—about half of him was lost. Stuart gave Dresden this pistol. At Lindquist's house, after Dresden summoned all of Lindquist's ghosts home, Dresden found Sir Stuart standing among the guardian spirits. When Dresden realized that Stuart had really given Dresden the symbol of his authority over the ghosts, Dresden thanked him. And then the gun turned into his staff. Dresden prepares the spirits to rescue Lindquist from Corpsetaker: the Grey Ghost. Stuart leads his soldiers and follows Dresden to the Big Hoods's hideout where Corpsetaker made her stronghold. He fought with zeal in the Nevernever against Evil Bob's defenses resembling WWII normandy landings[Footnote 1] on D-Day—becoming less faded. Stuart told Lindquist about Butters's spirit being tangled in Corpsetakers’s spell, unable to return to his body—Lindquist put Butters spirit back inot his body.
"'Tis a muzzle-loading pistol, boy. You have to reload them like a proper weapon."
Dresden: "They always never know fear.", Sir Stuart: "Nay, lad. Perhaps they were innocent of it once. But they proved to be quick learners when they raised their hands against this house."
"Hells bells. I'm bleeding ectoplasm?", Stu: "Ghosts generally do."
"Ye gods, but this city pales after but half a century, and I've lingered more than four times that."
- ↑ 1.0 1.1 Ghost Story, ch. 3
- ↑ 2.0 2.1 2.2 2.3 Ghost Story, ch. 50
- ↑ 3.0 3.1 Ghost Story, ch. 3
- ↑ 4.0 4.1 4.2 Ghost Story, ch. 40
- ↑ 5.0 5.1 Ghost Story, ch. 15
- ↑ Ghost Story, ch. 9
- ↑ 7.0 7.1 Ghost Story, ch. 43
- ↑ 8.0 8.1 Ghost Story, ch. 4
- ↑ Ghost Story, ch. 15
- ↑ 10.0 10.1 10.2 Ghost Story, ch. 2
- ↑ Ghost Story, ch. 3-4
- ↑ 12.0 12.1 12.2 Ghost Story, ch. 5
- ↑ Ghost Story, ch. 6
- ↑ Ghost Story, ch. 9-11