Fili hated her.
He heard them talking when the company was first put in the cells. He shook his head at his brother's comment about having something in his trousers. The blonde smiled, Cheerful despite the situation. He never changes.
Later he heard his brother telling her about the small rune-stone he carried. Kili told her about the promise to their mother. "She thinks I'm reckless," He said.
Its cause you are. Fili thought bitterly. He sat on the stone bench in his cell with his arms crossed over his chest and his feet resting against the wall. Huffing, he took one braid out of his hair then re-braided it.
When they escaped he hated her for being the one to save Kili. He hated himself for not saving his little brother when it was his job and he couldn't even do that. He was the one to bind his leg despite Oin saying it was his job.
In Laketown, Fili knew he was not going to let Kili be alone. Yes, he would have Oin and Bofur but they were not blood. Fili would not let Kili come to anymore harm.
The orcs came and Fili did his best to protect his brother but the elf was still the one to save not only Kili but the rest of them as well. She came in and slayed them all. Looking like an angel along with the blonde prince, she saved the small group that stayed behind. Fili hated her more when she said she was going to save Kili. It was his job and she was taking it once again.
When Bofur brought back the plant that Oin had asked for, Fili held his brother down despite his screams of pains. He saw the look of wonder in his brother's eyes as the woman healed him. He had only ever seen that look when the younger looked at him, their uncle, or their mother. They were the only ones who he loved. The ones he had always loved.
And in that moment, Fili knew.
Kili loved her.
He wasn't just his anymore.
He hated the she-elf for stealing his brother.
And he hated himself for letting him go.