Kaylee is the replacement for the ship's first mechanic, Bester. After Captain Malcolm Reynolds interrupts Bester and her having sex, he complains to Bester that ship needs to be fixed. Kaylee immediately shows that there is nothing wrong and fixes the engine instantly. Instantly receiving accreditation for knowledge of mechanics, Mal asks her to replace Bester.
After spending much time on Serenity, Kaylee is the shining personality among the crew. Her relationship with Malcolm Reynolds resembles a brother and sister. She, also, has a strong attraction towards Simon Tam, but he keeps his distance due to his proper upbringing though his feelings are mutual. Most of her knowledge comes from working with her father as well as her natural sense. Her explanation of it is that "machines just got workings,and they talk to her[me]."