I have to admit that to critique a commission means that I'll end up saying a lot of things that are directed more towards , since a lot of this would have to do with character design. So I guess I'll go through that first.
Though there are countless renditions of...uh, beast people out there (I've admittedly drawn a few as well), this still has a very original look to it. The four segmented horns are sweet, and the leggings and sleeves both seem to fit well with the rest of the clothing. The color scheme is nice and firey, with the purple wings standing out surprisingly well. After checking out the original it would seem that that credit goes to both of you, since Shiro changed a lot of colors on her own.
Back to actually ripping on just Shiro now.
The pose you can certainly be proud of. It has a mid-animation look that gives it plenty of energy, and you certainly didn't cut any corners when making this a full body commission. The hair wraps nicely, and the wings and tail look like part of the character rather than just being slapped on.The shading stays where it should be for the most part, though the fireballs don't appear to be admitting much light. I'm particularly confused by how one leg is completely darkened while the other is lighter than the rest of the pic, though it's still somewhat behind the smokey trails.
Speaking of those trails, they cross nicely, and give a solid impression that she was actually rising while creating the flares. The background suits this well, but you may be able to do without that large starburst in the bottom left corner. The stars in the top left corner are very interesting, I may end up creating a brush similar to that after seeing this.
Lastly, there's only one thing I would push you to fix. On the left of your name in the center, one of the starburst has a straight vertical slash due to some sort of layer mishap, I guess. It's an easy fix, so it doesn't really effect my opinion in any way, just pointing it out so you have something to curse at.
After all that bitching, I'll definitely be faving this. It's a well done character in a well done environment, and that gives this work a lot of impact.