He'd apologized. It may not have seemed like much but coming from Jayne, it meant the world. It meant that despite all their bickering and his talk of her leaving the ship, he cared. He didn't want to see her hurt. It meant he was ready.
In her mind, no one could replace Simon. He was her brother, her dopple, her knight in shining armor, her first love but brothers and sister couldn't be together he said. And she knew he loved Kaylee. The mechanic made him happy and that more than anything was what she wanted. So she let him go though they would never really be apart. They shared blood. Flesh of her flesh.
It was only when she began to let Simon go as a possible mate that she realized her reaction to the merc. She had no idea how long it had been happening. At dinner, on world, playing, showering, working out... especially working out, she watched him and marveled at the tight fluttery feeling she got around him. She knew that he found her attractive enough. Even if she wasn't his ideal. But physical reaction couldn't overcome his disdain for the trouble she brought with her and his fear of causing more trouble than he could handle. Tail wasn't worth the headache.
She also wasn't worth an apology before either. Not even when she and Simon confronted him on almost turning them in. If that had changed maybe the other had too. She would soon find out.
While on a job with the Captain and Zoe, River crept into Jayne's room and poked around. She'd done it a million times before of course but this time she had a specific purpose. Every picture, article of clothing, or dirty little memento from another woman she found she took. Leaving in it's stead the slightly crumpled comic she had made. There would be no other women for Jayne Cobb. She had decided.