In the background of certain levels, you can spot Mario's brother Luigi, who can be peeled off the level by Paperizing. If you peel him off in all 5 locations, he'll appear during the credits sequence. In addition, each individual peeling will unlock a rather amusing newspaper article in the leftmost house in Decalburg's Residential Area.

Location	How to unlock
Luigi Location	World 3-12: On a rock formation sticking out of the ocean in the background that's only visible from the pier.
Luigi Location	World 4-5: Near the end of the chairlift after the enemies stop coming. You only have so much time, so be quick.
Luigi Location	World 5-5: In the background to the left of the hot springs.
Luigi Location	World 2-5: On the fourth floor of the tower, in a window on the far right end of the second outdoor walkway.
Luigi Location	World 1-6: On the bridge leading to the central tower, look in the backgrond.