Kaylee is a bit of an anomoly in the
Firefly 'Verse. Kaylee is the nicest human being in said 'Verse: she believes in everyone, and could hurt no one.
Why, then, is she associating with such brigands as Mal and Jayne? Why, it is because of her slightly-erotic obsession with engines and all things that fly. Kaylee is not only the sweetest person around, she is also the most talented.
In the words of her captain:
"I don't believe there's a power in the 'verse that can stop Kaylee from being cheerful.Sometimes you just wanna duct tape her mouth and dump her in the hold for a month."