There are many returning characters from the Mario series as well as new characters included in Super Mario Galaxy!
Bowser, Mario's arch enemy has returned again, but instead of wanting to conquer the world, he's going for the whole universe!
 Bowser Jr.
Bowser's son, and apprentice. He is just as involved in collecting the Grand Stars and stopping Mario from getting them as his father.
Mario's brother, Luigi appears in the game at first just to help you out along the way, and sometimes he needs a little help himself! After you beat the game by collecting all of the stars, you can play through the game again as Luigi.
Mario, the hero, is set out to rescue Princess Peach yet again, but this time he must collect Power Stars and Grand Stars to help get to her.
 Princess Peach
The "Damsel in Distress" in the story, Peach is the Princess that you are out to save in SMG (and in most other Mario games).
 Princess Rosalina
Princess Rosalina is a new comer to the Mario world. She is the protector of the cosmos so if it has to do with space, she cares for it deeply. Her backstory can be found in the Library. As the game continues, more chapters unlock, revealing how Rosalina met the Lumas and became their caretaker.