I can only say a few things need improvement. (without specific detail sadly..)

I'd say probably head size, shape and body shape are what needs work. (I'd like to say your most recent comic panel shows what I'm trying to say pretty well stand alone.)  Surprisingly enough, (remember this is my own opinion) you draw Nyx in a perfect way, like correct proportions and stuff. I love your work, I really do, but that's all I can really say needs an improvement as I'm no professional.