The rumour of Luigi being in Super Mario 64 was one of the most talked about of the N64 era, but there'll be no fansite lies this time round because he IS in Galaxy.
We suggest you stop reading at this point if you don't want the surprise ruined for you.
Mario's lanky brother turns up in Haunted Mansion level in the game, seen in previous screens. The wimp appears to be trapped and, in contrast to the GameCube launch game, Luigi's Mansion, it's Mario that has to do the rescuing this time.
Will he be playable in later levels? We'll find out when we get our hands on the full game soon. Meanwhile, you can see the scene where Luigi appears for yourself in this video.