From the Super Mario Wiki
Yakkey is a talking key in Paper Mario. He is needed to open the door to Windy Mill. The key is locked away in a treasure chest in Tubba Blubba's bedroom. During the events of the game, Mario had to sneak past Tubba Blubba, who was sound asleep in his bed, and open the treasure chest containing Yakkey. Unfortunately for Mario, Yakkey was a talking key who was completely loyal to his master Tubba Blubba. As a result, Yakkey screamed as loud as he could, causing the "invincible" Tubba Blubba to wake up. Mario ran away with the key, and Tubba Blubba chased the plumber through his fortress. Eventually, Mario reached the windmill that held Tubba Blubba's weakness, his heart. When the game came to a close, Yakkey desired another master to guard his secret in the Windmill.