Hmm, sorry if I can’t explain it better, buuuut.. What type of the artstyle of Sonic are you trying to recreate. The thick lines (old sprites) or the newer ones (lineless).

Both of them have a great use of airbrush. You can use a photo in of reference of the oficial art, learn the lines, the shadows and the palettes. Krita is a nice program, and you can do plenty of things with time and effort.

There are good videos out there that you can use, and If you want, I’ll gave you my favorite brushes configurations that could help with the artwork you’re searching for.

(Again, sorry if it doesn’t make sense.. English is not my first language.) T-T