The Muscle/The Fixer
When we first meet him, Jack Armstrong seems like a dumb frontier guy with a gang (the Clary's Grove Boys). He seems like nothing more than a big dude who just likes to fight and wrestle. (Or "wrastle" as they used to say.) Abe meets him in New Salem, when Jack and his gang come to beat him up as a way of welcoming him to the neighborhood.
Except this new kid—Abe—is incredibly strong and tall and also a vampire hunter. So trying to beat him up is maybe not the best idea in the universe. Sure enough, Abe kicks Jack's butt instead. And afterwards, they become close friends. Of course. And, as it turns out, Jack is actually mature and thoughtful, which makes him the perfect companion to Abe (5.71).
As luck would have it, Armstrong becomes a capable vampire hunter, so much so that Abe feels comfortable sending him on his own when Abe is crippled with depression (7.9). But he's not just a strong, thoughtful guy—he also is very loyal. And he doesn't just take care of the business—he takes care of Abe.
When Ann Rutledge dies and Abe falls into a depression, Armstrong and the Clary's Grove Boys watch over their friend:
For the next two days, Jack Armstrong and the Clary's Grove Boys kept watch over him in round-the-clock shifts. They stripped him of his pocketknife and carpentry tools; took away his flintlock rifle. They even confiscated his belt for fear that he would hang himself with it. Jack saw to it that Abe's hidden stash of hunting weapons was moved well beyond his reach. (6.108)
So when Jack isn't busy saving Abe's life, he's busy keeping his secret. That's a friend we want having our backs.
Got all that so far? Armstrong: strong, thoughtful, a good vampire hunter, loyal. And on top of that, Jack Armstrong and the Clary's Grove Boys fix Abe up with some prime opportunities: they get jobs for Abe (6.4) and provide political support (5.68). So Armstrong not only is a help to Abe's vampire hunting career—he helps fix Abe up in his political career as well.
With all of those qualities, we almost want to read a book all about Jack Armstrong fighting vampires. (Though we suspect he'd be even better fighting zombies.) It's a shame that he dies in 1854 of some fever he caught while stubbornly struggling with a horse in the rain (9.19). In other words, he's got strong arms (even when he loses an arm fighting the vampire Joseph Nash McDowell) and a strong heart, but not a strong immune system.