It's a pretty picture and I like how soft it feels, but the girl in the black dress almost overshadows her companion through extravagance (lace, fancier wings, curly hair, and colors that brighten each other). My eyes are immediately drawn to the left with the stark (yet not stark) contrast of bronze-y yellow and black, and stagnate because of the strength on that side, which leaves me feeling a little empty because my eyes aren't being encouraged to go around and around the entire picture, as if there is no second person present. Of course, I do eventually see her, but almost immediately, I go right back to the left and the cycle starts once more.
As these are commission characters and not yours to change, perhaps eliminating the white background and making it something such as a very dark blue would be best, so the brightness of the right is seen.
Despite, I say again that I like the details, the soft feelings, cheeriness, calm portrayal of a darkening sky, and the over all design. Really just some issues with how one person stands out far more than the other, betraying the purpose of a picture that has both characters shown in an equal light.