Kaywinnit Lee "Kaylee" Frye (Jewel Staite) is Serenity's mechanic. With no formal training, she keeps it running with an intuitive mechanical talent. As a result, she's extremely sensitive to insults (or even candid observations) about the ship's appearance.
Kaylee has apparently always had an affinity with machines, and helped her father fix things on the planet ?. When Serenity sets down there for repairs, she encounters the ship's original mechanic, Bester. When he learns that "engines make her hot", he brings her aboard for an intimate encounter, during which Captain Malcolm Reynolds intrudes. When she proves that she can, within seconds, diagnose and fix the engine problem that has had Bester stumped for days, Mal makes a quick decision to dump him and take Kaylee on in his stead.