In New Super Mario Bros for DS he may have been Bowser's skeletal form fought in the second to last castle, but who is he in the other games in the series?
Because there's never really a good explanation for his appearance in things like Mario Kart games or spinoffs, or even the later Mario platformers. Heck, in the former he somehow appears alongside Bowser, and in the latter he appears despite Bowser never getting his skin melted off or even a scratch in the final battle.
So who is he? Is he a 'friend' of Bowser like Mario Party Island Tour implies?
Is he Bowser's skeletal form having gone through a time machine?
Does Bowser lose all his skin at random now?
What's going on?
Because there's never really a good explanation for his appearance in things like Mario Kart games or spinoffs, or even the later Mario platformers. Heck, in the former he somehow appears alongside Bowser, and in the latter he appears despite Bowser never getting his skin melted off or even a scratch in the final battle.
So who is he? Is he a 'friend' of Bowser like Mario Party Island Tour implies?

Is he Bowser's skeletal form having gone through a time machine?
Does Bowser lose all his skin at random now?
What's going on?