This is nice! Sonic’s sprite looks great here, and he plays as you’d expect. A couple things stand out to me that look visually odd to me though:

First, Sonic bobs up and down as he walks and runs. While this is consistent with Mario’s original sprite, it looks off for Sonic, who remains level in his walking and running animations in his own games.

Second, when tossing a fireball in midair, there’s a single frame where the top half of Sonic’s body will appear to “pop out” of his rolled-up sprite. If possible, I feel like it might look best to simply have Sonic remain rolled up when tossing a fireball in midair.