In the beginning of the game, when Kirby gets sucked into Patch Land by Yin-Yarn, he notices "a blue boy" that is being chased by an enemy. Kirby transforms into a car, and drives away with the boy. Later, Prince Fluff introduces himself, and him and Kirby become friends in an alliance to stop Yin-Yarn.
Prince Fluff looks almost just like Kirby in shape, size, and form. Instead of being pink, though, he is blue with orange feet. He is also wearing a crown, as he is a prince. He doesn't blush, either, as Kirby does. Other than that, only his personality is different, being a more serious, not as funny guy.
Kirby's Epic Yarn initially was not even a Kirby video game. In fact, during the beginning stages Prince Fluff was the planned protagonist and the game was going to be titled Prince Fluff's Epic Yarn. People within Nintendo, including president and Kirby co-creator Satoru Iwata, noticed the similarities between the two characters and saw an opportunity to switch Prince Fluff with Kirby, which would provide the game with a better chance of success. Good-Feel, the developer of the game, felt particularly close to the character they created, however, and decided to include him as the second playable character in multiplayer.