She was his little sister. Almost from the moment he met her, he felt an overwhelming need to protect her. Saw her sweet face and wondered what in the hell she saw in Bester.
Despite the large ranch he grew up on, back on Shadow, he felt a little alone with no one even close to his age. He had his ma and about a hundred older brothers in the form of farm hands, he had no sisters, younger or older, until he met Kaylee. And with her fixin' Serenity, he got a new member of his crew and the sister he had wanted in one fell swoop. His protectiveness even ran deep enough after that first meeting, after her parents said she could join the crew, he made a point of making sure that her bunk was next to his, with absolutely no lecherous intent despite the glares and thinly veiled threats from Zoe. Everyone on the crew adored her. She was their mei mei too. From Zoe's initial need to protect her to Wash and Kaylee inside jokes about the ship. Even Jayne, passed the crude comments, was softer around her. Inara took to Kaylee almost the instant he let out the shuttle to the companion. He was worried about introducing the small town girl to Inara, momentarily forgetting exactly how he had met Kaylee, but Kaylee worshiped Inara and her glamorous lifestyle. Inara, in turn found a little sister in Kaylee as well. He always felt that the reason Inara liked Kaylee so much is because of the girl's almost wide eye innocence and yet very blunt, but not unkind, nature in regards to Inara profession.
Mal had strongly enforced rules on his boat and no one dared to disobey him. Or at least he liked to pretend it was so. But even then, everyone knew not to give him orders on his own ship. Not only would it not be listened to, it would most likely bring several weeks of the worst jobs on Serenity.
But Kaylee was the only one who could get away with anything she wanted and never once anger him. She could pout or plead, flatter or even blatantly disregard. But it was Kaylee, his little sister, so it was alright. So when her and Simon started to date, for the lack of a better word, Mal knew not to even attempt his "on ship dating policy" speech. Eyes would be rolled and then it would just be flat out ignored.
Quite honestly, he was happy for her. Little Kaylee had been nursing a crush on the young doctor ever since she met him. And Simon would be good to her, that he knew. Now that Simon's own little sister was doing better and he finally got over his own initial shyness, and his foot in his mouth, Simon was busy now worshiping the ground Kaylee walked on. It still didn't stop Mal from borrowing Vera and giving the customary big brother "hurt her, you die" talk but he didn't actually hurt the boy which Mal felt that showed how good he thought Simon was for his mechanic. He hoped for the best for Kaylee, his mei mei.
But as she was his little sister, Mal also hoped for the love of God that they would choose the passenger quarters that were not right next to his to do all of their ruttin' in.
Mal covered his ear with his pillow and started to rethink his original standpoint on not hurting Simon.