A mercenary, Jayne works for Mal because he's the one who can pay him most. He's loyal, at least till something better comes along. Crude, thuggish, but with a sense of humour, and a great fighter with a well-hidden human side.
It's funny, Jayne is an asshole to my favourite character, and overall not a very nice guy, but he ends up being lovable anyway. Again, done in OpenCanvas on one layer, with reference.