Il Piantissimo is a human who is disguised as a Pianta. It is unknown why he does it, though. He appeared in the game Super Mario Sunshine for the GameCube. In the game, he offers Mario a Shine Sprite if he can beat him in a race. Mario races him 3 different times on 3 levels- Gelato Beach, Noki Bay, and Pianta Village. He resembles Koopa the Quick from Super Mario 64 who would give you a star if you beat him to a certain destination.
If you beat Super Mario Sunshine without collecting all the stars, you'll see a picture of Il Piantissimo looking at the paint-brush that Shadow Mario used to defile Isle Delfino.
If you remove it the Hat his face is of the Postman from The Legend of Zelda: Majora's Mask or the Runner of The Legend of Zelda: Ocarina of Time