Jayne Cobb is hired muscle. He and Mal met on opposite sides of a rivalry; Mal, while held at gunpoint, offered Jayne his own bunk and a higher cut than his current employer, so he turned coat and shot his then-partners. In one episode, he admits freely to Mal that he would have sold Mal out to an Alliance agent if the money was good enough. He is someone who can be depended on in a fight. He tends to act like a "lummox" who thinks he is the smartest person in space, but occasional hints of intelligence peek through this façade, giving the impression that he acts dumber than he is.

As Whedon states several times, Jayne is the man who will ask the questions that no one else wants to. Even though he is a macho character, he has shown a particularly intense fear of Reavers, more so than the rest of the crew. Despite his amoral mercenary persona, he sends a significant portion of his income to his mother, again suggesting that there is more to his character than what he presents to the rest of the crew.