 Jewel Staite as <unk> Lee " Kaylee " Frye — the ship 's mechanic . In the episode " Out of Gas " , it is established that she has no formal training , but keeps Serenity running with an intuitive gift for the workings of mechanical equipment . Jewel Staite explains Kaylee 's character as being wholesome , sweet , and " completely genuine in that sweetness " , adding " She loves being on that ship . She loves all of those people . And she is the only one who loves all of them incredibly genuinely . " She has a crush on Dr. Simon Tam . Kaylee is the heart of the ship : according to creator Joss Whedon , if Kaylee believes something , it is true . 
