Personally I'd put the character off to one side on the picture, down in the bottom right maybe (not RIGHT in the corner, but off centre). It gives the eye a bit more to do, and if the character is facing left the eye will tend to follow it off into the picture, to the background (at least it does in my case). I'd say a plain, dark background may be better, as well, but don't oversaturate the character or it'll look luminous.
