Cute and clever. Smooth controls. I like the personified shadow main character.
It took me a few tries to figure out what I was supposed to do (in the beginning, I just kept dying!). But I got the hang of it soon.
A few small suggestions:
1. Sound. This seems minor but it really does help immersion, and it doesn't take much.
2. A button to restart the game after dying, without having to refresh the page.
3. An onscreen indicator of how close the shadow is to "losing."
4. A scaling difficulty scale, so that it starts easy-ish, but later rounds are harder. (If you were already doing this, apologies; it was too subtle and I missed it.)