I'm struggling with the flying unicorn...cross between a unicorn and a pegasus? The scale of equine to human miiiight work because it looks like your intent is that the unicorn is a colt. I think you should try landing the unicorn...even if you keep the wings, and have it standing just higher than knee deep in the water...this would give you a cool reflection that would blend nicely with the color splash coming from the flowers lower left. If you start your eyes at the front left flower and follow the natural visual path it goes smoothyl around the curve and then right out the top of the picture as it follows the edge of the far wing. Makes the composition feel herky jerky because I have to bring my eye back dow to consider the girl. As to color and lighting you might try using the unicorn itself as a kind of light source radiating around the two central figures. The mountains and trees and what not are nice but they are staging and so we don't need detail from them. The stark color in the girls dress creates a sharp contrast with the unicorn...I'd try something either white or pastel so that from a color standpoint they seem to "agree" with each other rather than clash. Those are my initial thoughts anyway...