Hoban "Wash" Washburne is hired as a pilot by Mal, but the second-in-command, Zoe, initially does not approve, saying she simply doesn't like him. This changes since the two are married by the pilot episode, of course - and Wash is killed in the film, Serenity.
Serenity's mechanic. First joined the crew when Mal discovered her sleeping with Serenity's former mechanic on the engine room floor, and realized she was far more knowledgable about ships than the current mechanic was.
The hero of Canton, the man they call Jayne... he joins the crew of Serenity in a hilarious fashion. While holding up Mal and Zoe with his own crew at gunpoint, Mal and Zoe manipulate him into turning on his crew, explaining that the amount of money he's given and the quality of his room and board are completely inferior to what they offer their crew. Jayne more or less remains faithful to Serenity and Captain Mal.