Name that character!

:mario:
I exist I constantly tag along with Mario, and my fave color is blue.
 
Mario's overalls no idea.

I'm Petey Piranha's partner in Mario Kart: Double Dash!!
 
King Boo???

I have twenty-eight forms, based off of letters of the english alphabet, with two additional forms for punctuation.
 
Misty said:
King Boo???

I have twenty-eight forms, based off of letters of the english alphabet, with two additional forms for punctuation.

Correct.

Unown.

I am the third enemy species seen in Super Mario Bros.