I'm not familiar with Portal 2, but I can comment on the drawing.
First, it looks like you need some anatomy practice. Possibly it's the angle, but her eyes look somewhat to far up her head. Did you have a reference for the pose? Even if you have to find a separate one for the character and the pose, it would help to show you how the features adjust with perspective. I would also add that an upward-facing angle like this is really tricky.
Second, you could really use some more contrast for the lighting. I really can't see half her features because there's no shading. Strong lights mean strong shadows, generally. References are good for this, too.
What a beautiful piece here ♥ On the whole i must say the composition is really interesting ♥ Your colouring technique is not bad either ♥ I really appreciate soft colors ♥ However, it may lack depth at some parts (especially her face) but it's no problem with your style. The background is very simple, yet adds a great touch of lightness to the final thing ♥ My only negative comment may be about the girl's mouth. Actually it's not really aligned with the rest of her face so it might look weird and disturbing, as the rest of your piece is really well done. On the other hand, her chest looks amazing to me ♥
The hair is really well done. In terms of shading and looking up, it might be an idea to google image search "girl looking up" Try going over the outlines of a photograph like that, then delete the photograph, leaving your line art behind. Then slowly morph your line art into your image of chell, and hey presto, you've just challenged yourself and created something new.