Luigi

Luigi is Mario's younger brother and a playable character. He is much faster than Mario, making him good for speed run comet stars, but he tends to slide around a lot more. This is why Luigi is not recommended to use on missions, that involve many narrow platforms with no railings.

Meeting Luigi
he apears at a galaxy Luigi is a playable Character in some Galaxies. If you see him at the start of a mission, talk to him to play as him. However, once you complete the mission, you will have to return to playing as mario. if you Beat Bowser in World 6 you can play as him permanantly and use him in any world and galaxy. His power up costumes may also looke different from Mario's. Whenever Luigi completes a mission, a staff ghost is unlocked. This ghost will show you the location of hidden 1-ups, hidden power stars, and comet medals.

Trivia

 * If you save and quit the game, while you are still playing as Luigi, the start screen will show Luigi flying into view instead of Mario.