I'd question if Sonic the Hedgehog was 'designed' at all. The scattershot approach to media and branding they've taken over the decades feels largely reactive.
It has definitely been an uneven and chaotic ride compared to Nintendo's handling of the Mario franchise, though. Scattershot, indeed.
It's an interesting contrast in other ways too.
Mario is something close to a blank slate with no real personality. He's basically just a "seal of quality" - if it's a Mario game, you know it's a game that has received Nintendo's full attention to quality and is going to be accessible to all ages. You don't ever really "like" Mario. You like Mario games.
Whereas Sonic is an actual character, albeit one that has been developed in wildly inconsistent ways.
What's really baffling to me is the uneven quality of Sonic games. There have been some downright bad ones. You'd think Sega would have viewed him as their crown jewel. But there are a lot of times it felt like they were cranking out bad Sonic games just to stay afloat or something. It is almost impossible to think of Nintendo releasing bad Mario games.
https://en.wikipedia.org/wiki/Sonic_the_Hedgehog_(character)...
As another HNer quipped, though: Sega's uneven stewardship of that franchise sometimes makes you wonder if there's any plan at all!