In "Super Mario Galaxy" Rosalina is a princess tasked with protecting the galaxy and keeping watch over star-like creatures known as lumas. Her backstory is told through an in-game storybook. Each of the nine chapters are unlocked throughout the game.
The storybook begins with Rosalina as a young girl when she discovers a luma in an old spaceship. Rosalina and the luma fix up the spaceship and go on a search through the galaxy to find the luma's missing mother. Eventually, after failing to find the luma's mother, the two settle on an ice comet and build a home for themselves. Soon other lumas began appearing and Rosalina becomes the adoptive mother to them all. The comet traveled the galaxy searching for lost lumas and visited Rosalina's homeworld every 100 years.