Kaywinnit Lee "Kaylee" Frye is a fictional character from the short-lived television series Firefly and the movie Serenity , portrayed by Jewel Staite. Kaylee has exceptional mechanical aptitude, despite her lack of formal training, and serves as ship's mechanic to Serenity. Kaylee is considered by the crew to be exceptionally sweet and chipper, the type who maintains a bright attitude even when others are feeling low.