Personally, if your going for an adventure-esc Sonic, I'd recomend chaomix's video on how he makes his thumbnails. That pretty much explains a lot of things like his smile, his shoes, proportions, etc. The main critique I have are the proportions for this piece. Personally, I start with a circle with another smaller circle directly underneath. Try not to make the limbs too thick. For the spikes, there are three main ones with a side view. Front view can be ditto since trying to do them logically isn't the easiest of tasks. Back view is 9. There's two slightly smaller spikes on his back. You can just draw one with side view. For the shoes, just imagine an Oscar Mayers Hot Dog. That's the proportions of his shoes. You can thin out the ends if you want. The smile is just the Nike logo. Other than that, this is really good. I really like your work, especially your Pico. Great job! 
