First of all I would just like to say that Nepeta is one of my favorite Homestuck trolls and I am happy to critique this.
For the most part you seemed to get the proportions right. The head seems a bit big, but that might just be due to the cut off shoulder. I like her expression... It seems kind of devious. It's a very interesting choice in comparison to what I have seen people making Nepeta look adorable. Speaking of her face... I must commend you for realizing that the nose and cheeks hold a slightly darker reddish shade in human anatomy and applying it to the drawing... It makes sense to have her that way. But it seems a bit overstressed. Unless you were going for a blush effect, I would possibly consider making it more of a green-gray... To match her skin a bit better. But yeah... The shading seems to be done where it should be. And it isn't overdone like how some people might do it. I like how she stands out of the background. It makes her easy to see... Um... Not much else to say. Great job.