This looks amazing! The movement is so fluid, and everything stays proportional; it was definitely worth the time :P

Hey! Just thought I'd leave some feedback. Something I think is really holding back your work is your faces. The body is well proportioned and honestly doesn't look too bad, but the eyes are looking in different places, and the mouth isn't centered.

A trick for the eyes is to make sure they're always horizontally level across the face, as it's impossible to look up and down at the same time unless you have a lazy eye.

For the mouth, it's popular in anime and manga to draw the mouth off-center, but they can get away with this because it's a visual trick that only works with very simply shaped mouths. If you're going to draw the upper lip, it tends to look off if it's not aligned with the nose. I don't think you'd have to change much, just move the mouth you already have here over a bit so that it's aligned with the nose.

Anyway, I hope some of that was useful!

Honestly, it looks really good as a sketch, but the line art is really holding it back. I would be curious to know if you're using a drawing tablet or a mouse. You list MS Paint as one of the tags, but even without that, this image screams MS Paint.

I'd recommend graduating from MS Paint as fast a possible. I recommend Clip Studio Paint, but as a free option, I hear Krita is pretty good. Of course, if you're just doing this for fun, you don't have to do anything you don't want to, but personally I find better drawing software to be more fun ;P