Someone was practicing in the training gym and Demyx wasn't going to lose the opportunity.

He needed to know more about the people around him, and what they could do, and if they were going to practice and put themselves on display, who was he to complain?

Demyx appeared out of nowhere in the corner and sat himself down on the floor, small and unassuming.

This person wouldn't notice him, he figured, he could observe and vanish when he was done.