Character Analysis
There's no denying that Johnny is a holy terror. When we first meet him, "the kid was in the street, throwing rocks at a kitten on the opposite fence" (7), and things only get worse from there. He torments Bill and Sam to no end and he seems intent of committing murder before it's all done. Even his own father will only take him back after getting paid. That makes him a fulcrum for comedy more than a character in some ways: an agent of chaos making our protagonists' lives a living hell.
At the same time, O. Henry does give Johnny some subtle depths that make him a bit more interesting than your garden variety delinquent. In simplest terms, he's a kid and his mayhem is particularly kid-based in nature. Johnny's essentially playing a big game of make believe with his captors, such as when he asks Sam to "let me play the Black Scout to-day" (45). And because they let him do it (instead of tying him up or threatening him), he lets his imagination run wild. He plays a bit rough, but he is quick to recant when Sam reminds him of his place:
'I was only funning,' says he sullenly. 'I didn't mean to hurt Old Hank.' (45)
Johnny seems entirely unaware of his predicament. He says:
I like this fine. I never camped before . . . (17)
A kid who hasn't gone camping? Who knows, maybe Sam and Bill are actually the closest thing to parents he's had—it certainly doesn't seem like he gets much love at home. Bits like this give Johnny a bit of depth, as well as help us recognize universal traits of kid-dom in him. That makes the hell he unleashes all the funnier, and maybe even something we can relate to in the process.