Many people thought that Rain was an emotionless person. That she always had the same poker face and never showed any emotions, but they were wrong. Utterly wrong.
Elliot noticed it for the first time during the concert. Frustrated by Rain's ignorance and her keeping distance from him, he could not stop himself from staring her. He could not bear to take his eyes away from her. It was as if he was compensating for the recent few days where she kept her distance from him by staring at her. That was when he noticed her expressions for the first time. Although at that time she was smiling genuinely, but more than her face her eyes echoed the happiness that she was feeling.
Her eyes were always fascinating for Elliot and were the first thing that had enticed him towards her. When he noticed her eyes that day, they appeared to be smiling to him. A genuine happy smile.