I'm so....alone....

Ahh forgot to mention i don't really wanna put this on an actual critique thing..but here we go! ^^

Let's start with the perspective. I really the perspective you used for this because I can see the great overview of the background as well as the details of the character such as the design and emotion. I think you should keep pushing it tho, it's a bit hard to tell if the focal point is on the background (the sky) of the character itself (especially the eye color that sticks out). To me I think you want it be the background. So that means you don't need to put all of those details in the character because it's not the focal point. I also think you should change the characters perspective as well. So instead of the side angle, try a 3/4 angle so that you'll mostly see the back of the character's head and a little bit of the eye, teas, and mouth and still show it's expression.
The tree had a nice angle to it, it kinda seems like you angle it just so that it fits the character in nicely. Try to avoid this in the future. Remember in compositions like these, it's always background first, then the character. ^^ The tree should a nice point in between it's branches and the trunk. I like the branches on the end give the perception that it's very small, that's good! However, splitting the branches off (at the bottom right corner) kinda takes that away. Along the angle, it's hard to imagine just how the tree stand and looks by itself. The branches here makes it seem like it's huge rather than, just the character being small.
I absolutely love the lighting in this pic! <3 You did a wonderful job matching it with the background. I think you should make it a little darker tho. Try to cover up all of that white on the character, and maybe just leave little tips of white for the highlights. The gradient shading was a nice touch and i think you should include normal dark cell shading to complement it. The character seems a bit flat in areas with just grads (without the erasing). You should for thee tree as well. While I like that the gradient shading gave a good perspective (on the bottom left) the rest of the tree fell short of it. One way to fix this is by the texture you used for it. Your going to need waay more lines for trees, but it was a good start! It seems like you left out a couple places like right under the character (by the scarf) and  the bottom left against the darker gradient.  I can understand you doing this because of the lighting but you should use the lines AS the lighting. Draw the lighter lines near the the light source and the darker lines away from it. This tutorial really helped me with my trees ^^ tephra76.deviantart.com/art/Tr…
Also don't forget the put shading where the character is leaning against too.
The sky really is beautiful and i love the comet and all the stars and gradients you made to for it X3 I actually recommend you put even more stars (i now it's alot of work X')) one way i kinda cheat of this is by just focusing on a little area and fill it with dots and stars and just copy/paste them every where else in the sky. I also think you should make the gradient more details as well. Your on the right track with lighting. Try to include very little black the sky and just go layer upon layer with your soft brush and use all kinds of different colors, opacities, layer effects and everything you can to fill it up. It takes alot of experimenting, but you should fell that sky with as many details as you can (without overwhelming it of coarse ^^) but the trick is to make it seems big as the focal point of the pic, and both the tree and character small.
For the comet I think you should try some kind of motion blue (do you use sai or photoshop?) For sai I'm sure you can use the blur brush to help give you that effect or maybe even just a tiny soft brush. In photoshop i know there is an effect called motion blur that you can use too ^^
Lastly I think you should work on fitting in the character and tree with the background in unity. All of the black outlines look too hard and layered against each other. The way to get rid of this taking a soft brush of the background color and drawing over all the outlines and than later adjusting with opacity to give it a blur effect. (it's sort of a painting skill) but you can experiment with the opacity, colors, layer effects ect.

Hope this helps! -Jen