Lumine critique is pretty good! I think it's less about "making a good critique" and making an attempt to look at the drawing beyond the obvious, looking for the technical details you have learned so far. Which you did very well.
For works with characters I personally also pay close attention to their body language and expressions (since that's usually what I use for drawing), in this case her open and arms coupled with her happy/excited expression. The drawing would probably had a ver different feeling if she had her arms crossed or closed. Or if she had her arms completly open at her sides (like they do when they "fly" in the game). It's almost like she's a bit cautious but also excited.
Your self potrait is really nice too!