Tiara is a little girl that lives with her mom in a town in the Johto region. While she was growing up she had a Cleffa but she believed it had since become a star as its silhouette is like a star in the night sky and the Rotom Dex said that Cleffa are believed to be the reincarnation of a star.
She would go to the park and look up into the sky to see if she could find which star was Cleffa but it was often the daytime and she couldn't see the stars. One day, a group of Unown were in the park and noticed
Tiara longing to see Cleffa's star, so they made the entire town nighttime so she could look for it in the nights sky.
After an investigation by
Cynthia,
Ash Ketchum and
Goh to figure out that it was the the Unown that had made the town dark for a week, they defeated the Unown and before they left, they transformed into a Cleffa so
Tiara could see it.