Bubbles look really nice, it has a watercolor sort of feel to it. Watercolor effects in general look so nice.
People who can digitally paint /headshot
Wouldn't call it empty.... maybe 'grounded' or 'unfocused' if I had to search for a negative term? Maybe because there doesn't seem to be any point of focus. The girl in the back is looking at the girl in the front, but she's looking straight outside the picture. Whether through use of contrast (color, brightness, saturation, shapes, etc) or perspective or other devices, there doesn't seem to be anything that calls for my attention. So for a art piece, my eyes kinda wander. Food for thought.
Edit: though if I had to say, my eyes currently focus on the red haired girl's golden eyes, because well it's eyes, the general line of sight of the blue haired girl goes through them, and it seems to be an area of the most contrast and interest/motion seeing as the rest of the piece is mostly shades of blue and purple. eh