Disclaimer: The setting, Annabeth and Luke belongs to Rick Riordan. He is amazing. I'm not.
Annabeth loved him like a brother. With these words, he could feel his last will to live drain. Like a big brother. She had been his last hope, but there was nothing there. As expected.
He had felt that way about her too, once, he remembered vaguely. When they were younger, and he had saved her and protected her.
Always. That was the word. Always. If it was up to him, he would always be there to protect her.
Every time she was in danger he had spared her. That was how he knew he loved her. How he knew he felt more. He wasn't her big brother anymore. The love he felt for her shone through Kronos every time.
Because of her, a part of his heart was still pure, flooded by love.