KAYWINNET LEE FRYE is the mechanic aboard Serenity. She first got the job when she was discovered having a tryst with the ship's mechanic, Bester. She fixed the reg couple, which had grounded the ship for half a week on its way to a job on Paquin and Captain Reynolds was so impressed with her know-how he hired her on the spot. She went off to ask her folks, and has been aboard ever since.

Kaylee is irrepressibly cheerful, and has an innate knowledge of machines and how they work. Prior to her job on Serenity, she worked with her father when he had work. The quickest way to get on Kaylee's bad side is to malign Serenity. She dotes on that ship, and say one bad word about her baby, and you're likely to get attacked by Reavers, snatched by hill-folk, or stabbed a whole lot. In her off-hours, she enjoys spending time with Inara in the Companion's shuttle, and the two are close friends. Nothing seems able to keep Kaylee down for long. Even when she is sent on scut-work missions, she'll find a bright side, be it taking on passengers or new used synchronisers. She is passionately loyal to her captain, whom she adores, and she is treated as resident little sister (méimei) by Mal, Inara and even Jayne.

Unlike the rest of the crew, Kaylee has a hard time handling herself in a firefight. She is willing, but not quite yet able to overcome her fear of combat situations. She is most at home in the engine room, and can usually be found wearing a pair of grimy canvas coveralls over some shiny flowered or spandex girly top. Kaylee likes ruffles and bows, and (appalling) layer-cakes of dresses of questionable taste. She's awed by Inara and Simon's fancy manners, but also sees them as having little place out in the black. Kaylee is no shrinking virgin. She appreciates good old fashioned sex and has few or no hang-ups. However, she's only had her eye on Simon since he and his sister stepped on board, and she seems willing to wait for the good doctor to make the first move. She has grown very fond of River, and the two have a very sibling-like relationship. Kaylee was badly spooked when she witnessed River shoot and kill three of Niska's men in cold blood. She knows that River saved her life, however she was frightened by the change in River, and how the disturbed young girl likened it all to a game.

Kaylee has three passions: engines, strawberries, and Dr. Simon Tam. A girly-girl grease-monkey, she has a crush on Simon and from the moment he set foot onboard, she has been determined to draw the shy and reserved young doctor out of his shell. At first frustrated with his clinging to propriety that may have its place in the Core, but she sees as useless out on the Rim, as the two get to know one another a genuine closeness develops between them. It may not be progressing towards romance quite so quickly as Kaylee would like, but the Doctor is definitely fond of her, and the two have shared one or two potentially compromising moments in the 11 aired episodes.