Kai stood there silently for a while as he stared at Aria with a complicated gaze. He didn't know what to do at that moment at all.
Was he supposed to get angry? Was he supposed to take that picture from her and destroy it or was he supposed to simply let her keep it since it had been in her possession this whole time even before they grew apart? It was such an alien situation for him.
Aria, on the other hand, was panicking inwardly. She was cursing herself for allowing Kai to find that picture while also fearful of what he would do. That picture was supposed to be something he completely forgot existed.
In a sense, she hid the fact that she still owned something from that time with her Kai and that could be considered a form of lying. Perhaps Kai all the hard work Aria had put into trying to fix their relationship was now completely ruined.