Will Mario’s faithful sidekick Luigi make an appearance in Nintendo’s spacey sequel Super Mario Galaxy 2? Well, if a newly released screenshot from the game is to be believed, the answer is simply: yes.
The extent of Luigi’s role in the game isn’t known, and until Nintendo officially announces Luigi’s appearance, don’t break out the “We Love Luigi!” banners just yet. (But keep them ready just in case.) Andrew Pfister is on hand at a Nintendo event as we speak, so if he finds out something, I'll let you know.
Source: Nintendo Everything