^While I agree with you, North, I don't think Shadow is the best example since he is a major character that is wanted by fans as well (Even though it think Sonic, Tails, and Knuckles should be first, and even then I only want one third-party character from each franchise)
I think a better character to compare would be Waluigi or Birdo with the Mario series. Despite being fan-favorite characters, (Well, I don't know much about Birdo on that subject) they have only appeared in spin-offs and have had minuscule roles. Not to mention that Nintendo itself sem so ignore all of the fan-feedback for wanting see more major-appearances from them.
Now, the solution to this problem would be someone like Toad. His race is arguable the most common Mario species, he has had major roles in main games, and he's a fan-favorite. Having a character like Waluigi taking away someone who has had a rich history of success and is also a fan-favorite to boot would feel very unworthy and cheap. An even better example than Waluigi would be Geno, but I'm not even going to go there.