From the Super Mario Wiki
The imposter Millennium Star is a silver, mustachioed star who will grant the wish of any player who passes his test. The Millennium Star fell on Peach's Garden while Mario and his friends were resting. Mario and his friends argued about who would get the star, which resulted in Lakitu coming and suggesting that they have a contest to see who get the star.
When the player first meets Princess Daisy during the story mode, Daisy makes the Millennium Star fall in love with her. When the player beats Waluigi's Island, the Millennium Star then says that the player has one more opponent. The player doesn't understand so the Millennium star tells the player that it is him. The Millennium Star then challenges the player to the Stardust Battle. After the player beats the Stardust Battle, the Millennium Star then says that he isn't the real Millennium Star. Tumble then opens the top of his die cap to reveal another, much different Millennium Star; Mario and his friends had been trying to please was an imposter. The real Millenium Star is a white star that emits an intense rainbow-colored halo and truly has the power to declare the Superstar of the Universe.
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