Jayne is a mercenary who did not fight on either side in the Unification War. He met up with former Independence fighters Malcolm Reynolds and Zoë when his gang tried to rob them. Mal offered him a larger share of heists and his own bunk, and so Jayne shot one of his companions and joined up with Mal instead.

Jayne is good in a fight, and reacts before he thinks on many occasions. Although he often appears to be somewhat stupid, he has his own peculiar wit. He has played successful jokes on the more intellectual Simon Tam (for instance, tricking Simon, who harbours an excessive fear of the vacuum of space, into putting on a spacesuit to board a pressurized ship in "Bushwhacked"). He's also credited for the witty, fan-favourite line, "I'll be in my bunk,". Jayne also has a keen sense of danger and is often the first to raise an accurate warning about impending peril (also in "Bushwhacked").

Although often represented as a strict mercenary, Jayne has his own code of honour. He is loyal to his employer as long as he is being paid well. When he has not acted up to his code of honour, he has worked to atone for it, for instance by buying a crate of fresh apples and sharing it with the crew of the ship for what he did in "Ariel". Jayne also demonstrates unexpected sentimentality in "The Message", when it is revealed that he sends some of the money earned by his mercenary activities home to his mother. In the same episode, he proudly sports a ridiculous orange knit cap with earflaps, simply because his mother made it for him.

Jayne keeps a large arsenal in his bunk on Serenity. He has named his favourite and most powerful gun (a "Callahan full-bore auto lock with customized trigger double cartridge thorough gauge") Vera. He acquired it after killing the previous owner, an assassin trying to kill him. Jayne frequently carries a handgun based on a LeMat Percussion Revolver.

Jayne doesn't have many fears, but he has shown fear at anything to do with Reavers, a subpopulation of feral humans. He is convinced that fugitives Simon and his sister River Tam will bring the Alliance down on them, and this concern fuels his dislike of the Tams. Jayne has also demonstrated fear of dying in what he considers an unmanly manner, notably being "spaced".

Jayne is regarded as something of a Robin Hood-like folk hero by the people of Canton on Higgin's Moon ("Jaynestown"). They have erected a statue of him in the centre of the town and sing a song dedicated to "the hero of Canton, the man they call Jayne" ("The Ballad of Jayne Cobb") in which they tell how "he robbed from the rich and he gave to the poor". The true story is, however, less heroic. Jayne and his then-partner Stitch Hessian did indeed steal from Magistrate Higgins and drop the money on the town, but this was only because during his escape, Jayne's ship was attacked and damaged and he needed to lose a lot of weight from the ship just to keep it flying. Jayne dumped the money in desperation, but only after having already thrown out his unfortunate partner.