Kaylee is considered by the crew to be exceptionally sweet and chipper, the type who maintains a bright attitude even when others are feeling low. Kaylee joined the spaceship Serenity when its captain Malcolm Reynolds found her having sex with his then-mechanic, Bester. When Bester informed Mal that the ship could not be fixed, Kaylee quickly repaired Serenity and Mal invited her to take Bester's place.
In the pilot episode, Kaylee was shot by Alliance agent Lawrence Dobson. Simon Tam used her injury as a bargaining chip to be allowed to remain on Serenity with his sister, River. Kaylee developed a romantic attraction to him which remained unrequited during the series' run. A small note in this segment of the episode is that we see Jayne Cobb huddled on the catwalk outside of the infirmary when Kaylee is being operated on, implying the importance of her to him.
Kaylee is known for her affinity for strawberries. As part of payment for being allowed passage on Serenity, Shepherd Book offers her a small box of strawberries, which she eagerly accepts. The second instance is when she accompanies Malcolm Reynolds to a ball, at which point she immediately homes in on the bowl of strawberries on the buffet table.
Despite her mechanical aptitude which allows Kaylee to fix just about anything, she has a desire for girlish things and is annoyed when Mal does not see that she is a woman as well as a mechanic. Although most notable in the episode "Shindig", this recurs throughout the series. The overall relationship between Mal and Kaylee closely resembles that of a brother and sister, which may explain Mal's reluctance to acknowledge Kaylee's femininity and sexuality. When, in the movie Serenity, she tells him of her longstanding sexual frustration and occasional resort to mechanically-aided masturbation ("...goin' on a year now I ain't had nothin' twixt my nethers weren't run on batteries!"), he responds with anguish, "Oh, God! I can't know that!"
When her ability as a mechanic fails her, she tends to lose her sunny disposition, as happened in "Out of Gas". Kaylee tends to talk about Serenity as if the ship were a sentient creature, and Kaylee takes insults to the ship extremely personally.
In the movie Serenity, Simon Tam confesses his feelings to her, which she reciprocates; the ending strongly implies they are now a sexually-active couple.
I am not Kaylee, but I am using her character for fun