Original: Whedon, Serenity: Director's Commentary, track 7 "Mr. Universe" Even though he is a macho character, he has shown a particularly intense fear of Reavers, more so than the rest of the crew.
Simplified: Whedon, Serenity: Director's Commentary, track 7 "Mr. Universe" Kaywinnit Lee "Kaylee" Frye, played by Jewel Staite, is the ship's mechanic.