You don't need to do backgrounds if you don't feel like it. A white bkg can be very powerful.
The most important is to use a background color that complement your main drawing.
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In this case here, Pyro's Brownish skin and hair are not in sync with the green.
[Might advise to try out something that closes the idea of fire?]
On the other hand with your sonic's previous pic, the Blue is clearly detached on the Yellow hue. Harmony's not an exact science, but it's worth a shot to look up and try out if you've got some time. You might surprise yourself.
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The second problem is the negative space.
There is a lot of empty space left and right, and it lowers the intensity of the subject.
Again, your previous "Final Design" is much more powerful: we see the guy right away and we get what he is about. Again, this is not an absolute truth, but most of the time being as direct as possible makes the storytelling efficient.