See: Rosalina's Storybook
Super Mario Galaxy Edit
In Super Mario Galaxy, after he finds all three Star Bunnies, Mario meets Rosalina at Gateway Galaxy. After collecting the Grand Star, Mario agrees to help Rosalina collect the Power Stars to power the observatory.
Super Mario Galaxy 2 Edit
While her role in Galaxy 2 is limited, she has her appearances.
Rosalina appears briefly at the end of the main game, where the Comet Observatory flies by Starship Mario and she talks to Mario.
Roles in other Games Edit
After her debut in Super Mario Galaxy, Rosalina had a plethora of cameos and appearances, starting with her appearance as an unlockable character in Mario Kart Wii. She could be unlocked by playing ten races with a Super Mario Galaxy save file on the system. A Luma would race alongside her. She would also be unlockable characters later on Mario Kart 7 and Mario Kart 8. She was also adownloadable character in Mario Golf: World Tour.
In Super Mario 3D World, after you beat World Star-2, Rosalina became a playable character. Her special ability was a Star Spin that could be used as long as she didn't have a power-up. She was the slowest character.More recently, she is a playable character in Super Smash Bros for 3DS and Wii U, where she would fight alongside a Luma. She has her own stage, too: Mario Galaxy. Check out her trailer