Kaywinnet Lee "Kaylee" Frye is Serenity's mechanic. She is an exceptional engineer and has a natural intuition for machines, despite her lack of formal training. She is kind, trustworthy, sees the best in people, and is known as the Heart of Serenity. Kaylee had apparently always had an affinity with machines. On her home planet, she would often assist her father in repairing things.
Kaylee joined the crew of Serenity when its captain, Malcolm Reynolds, interrupted her and Bester having sex in Serenity's engine room. At the time, Bester was Serenity's mechanic. However, when Bester incorrectly informed Mal that the ship could not be fixed, Kaylee quickly proved him wrong by diagnosing the problem and repairing the grounded Serenity. Impressed with her mechanical ability, Mal immediately offered her Bester's job. Kaylee happily accepted, and left her family (and Bester) behind to join the crew.
Kaylee was shot in the stomach by the Alliance agent Lawrence Dobson. Simon Tam used her injury as a bargaining chip to be allowed to remain on Serenity with his sister, River. While Simon tended to Kaylee, Jayne Cobb was huddled on the catwalk outside of the infirmary watching the surgery. Earlier, Jayne had been banished from the dinner table for making lewd comments about her obvious attraction to Simon, thus humiliating her in front of Simon while leaving Simon himself very embarrassed. And yet, his obvious concern emphasized her importance to him and implied how deeply the other members of the crew care about her—although Jayne would still embarrass her later by making fun of her attraction to Simon, usually right in front of Simon.
Kaylee had shown an affinity for strawberries. As part of payment for being allowed passage on Serenity, Shepherd Book offered her a small box of strawberries, which she eagerly accepted. Later, when she accompanied Malcolm Reynolds to a ball, she immediately homed in on the bowl of strawberries on the buffet table.
Along with her mechanical aptitude, which allowed Kaylee to fix just about anything, she had a desire for girlish things and was annoyed when Mal did not see that she was a woman as well as a mechanic.
On the rare occasion that her ability as a mechanic failed her, she tended to lose her sunny disposition, as happened when Serenity's engine failed after an explosion, leaving Kaylee saddened that her ship had become so damaged and she hadn't noticed the 'warnings.' Kaylee tended to personify Serenity, talking about her as if the ship were a sentient creature, and Kaylee took insults to the ship personally when someone referred to it as "junk."
River revealed that the attraction between Simon and Kaylee was not a simple crush of infatuation, but that the two were in fact, in love. Simon Tam later confessed his feelings for her, which she reciprocated. The two finally consummated their relationship; Kaylee and shirtless Simon kissed each other passionately before sinking into her quarters, with River peeking in wide-eyed from an overhead hatch.