He knew had a strange feeling inside him since Kairi came to the islands. He never knew what it was truly called.
Every time he saw her, the blood rushed to his face. Every time he heard her laugh, he wanted her to never stop. When ever she complimented him, or hung out with him, his heart felt a comfortable warmth.
He wanted to run his fingers through her wine red hair. Hold on to her and never let go. Kiss her soft, peach pink lips. His heart burned with a strange feeling for her. He couldn't figure out what it was called.
There had been a time when that feeling led him to give up his heart for her. He saw her fast asleep, doomed to remain in that state forever. It broke his heart in two. He found out that her heart had been inside him the whole time. He never lost her. She had always cheered him on. He wouldn't live with himself if he never saw her awake again. That feeling he had took control. He gave his heart away to return hers.
As he stabbed his heart, he did not feel any pain. The only thing he felt was that same warm feeling in his heart. When he saw Kairi wake up, he smiled, knowing that he gave up his life for the girl he loved.
Later, she trusted him with her good luck charm. He made a promise that he would return it after he saved his best friend. The feeling inside him grew so much he did not know if he could keep it calm. His heart felt the same warmth he had whenever he was around her. To know that she trusted him enough to give him her charm made him feel happy inside.
Since that day, he finally figured out what that strange feeling is. Why he felt it only around Kairi and nobody else, why he gave his heart up for her alone, and why he felt searing pain when he heard Kairi couldn't wake up.
He figured out that the feeling is called love.