It may be a little late to ruin Super Mario Galaxy for some...but I thought I'd be considerate anyway. So, if you collect all 120 stars in SMG then beat the game, you're then given the option to play as Luigi.
Finally, Luigi gets his own time in the spotlight! Yeah...there was Luigi's Mansion. And he's still riding on his brother's coattails, but you gotta love the guy! The cowardly, gangly, unpopular, invisible, younger brother who'd rather sit back and let his brother do all the platformin just has an incredible charm to him that I just love. He doesn't want to be a hero and here he is, thrust into an adventure to save the galaxy!
Once you defeat bowser as the final boss, with all 120 power stars, an aditional cutscene will be added. I didn't understand it much, but it looked like rosalina left the earth with the planetarium, and in "play room" appears another luma, left behind by her. Then, a note appears and it says: you can now play as (luigi's face) LUIGI! and at the same time, you hear luigi's voice saying: "Super LUIGI Galaxy!" Then you'll go to the file selection screen and you'll see an adittional button which shows mario and luigi's heads, and you can selct to play with any of them. If you choose luigi, inside the same file planet, you'll open another file in which you can play as luigi, and also, you can select luigi as an icon for planets.
Playing with luigi is alot better: luigi runs faster (tough it's more difficult to brake), his jumps are higer/have a longer range (long jump), and when you talk to the postman toad and he gives you the letter(peach's one), it'll still be for mario, but after reading it, he'll say "oh, it's from mario..." but don't get angry. and instead of having 5 1-up mushooms, you'l have.... 20! Also, when you're been luigi, since you're taller than mario, part of the normal suit can be seen uncovered by the bee one. and when you're rainbow luigi, you'll go super fast when you get the invincibility speed boost.
Luigi takes a little getting used to. He skids all over the place, so you have to really be careful near edges and narrow platforms. He also seems to jump a little higher than Mario , but after a little bit of time, controlling Luigi is like second nature.
Wow...that was less than flattering! Still, Luigi is such a lovable goof that there are times that I feel he has more personality and depth to his character than Mario does. I wonder why Luigi has been regaled to play the role of the cowardly younger brother? Could it have been due to some residual influence from the Super Mario Super Show?