Yakkey

Yakkey is a talking key in Paper Mario. Yakkey's name is a portmanteau of "yak", which means to talk non-stop, and "key".

Paper Mario
In Paper Mario, Yakkey is needed to unlock the door to Windy Mill. The key is locked away in a treasure chest in Tubba Blubba's bedroom in his castle.

After Tubba Blubba goes to sleep in his bed, Mario opens the treasure chest containing Yakkey. Unfortunately for Mario, Yakkey is a talking key who is completely loyal to his master Tubba Blubba. As a result, Yakkey screams as loud as he can, causing the "invincible" Tubba Blubba to wake up. Mario runs away with the key, and Tubba Blubba chases him through his castle. Eventually, Mario reaches the windmill that holds Tubba Blubba's weakness, his heart. He uses the key to open the mill, and defeats Tubba Blubba's Heart deep below.

Later in the game, Yakkey can be found by the well in Windy Mill desiring to guard a new secret, hopefully a happy secret such as a present.

Tattle
Goombario can use Tattle on Yakkey when the latter is by the well inside Windy Mill after Chapter 3.
 * "This is Yakkey, a magical key. It was lucky for us that it spilled the beans about Tubba Blubba's secret. But I wonder what it's going to do..."

Trivia

 * When Mario is opening the door to Windy Mill, the game refers to Yakkey as "Mystical Key." He is also listed as the "Mystical Key" when viewed in the Key Items menu.