Name that character!

Since none of you are able to pull this off, I'll reveal the answer.

It's Baldi
 
Yeah, we're officially out there. Although what hints you gave us were kinda vague.

Anyway, this character has no reason to be in Mario Kart but is not a clone.
 
Sounds like something that would depend on the answerer's opinion, but I say that applies to all the non-Mario characters.

This character has a line of text during the ending of Super Paper Mario that could be mistaken out-of-context for a Waluigi yell.
 
Oops yeah you're right I should've said instead "Is not a main Mario character but appears in a main Mario game". Yeah should've said that.

I will agree though the non-Mario characters have no reason to be here but that doesn't necessarily mean their inclusion is bad. I play them all the time!

Anyway the character you're talking about...

"Waaaah"

The only cry I can think of is Nastasia.
 
Woah it was kinda out there. And yes Honey Queen is correct.

Anyway, this character was the one who people were the most surprised to see playable in the latest(?) Mario game.
 
Sonic, because he has appeared in games, so therefore he is canon :D

This Character is the boss of World 2-Castle, 6-Castle, and Flower-12 in a 3-D Mario game about cats.
 
Boom Boom is correct. My guess is E.Gadd.

This character is dangerous but useful. You use him to kill a boss when he pops out of rocks the boss throws at you. You find him in a galaxy trotting game.