Search the indicated locations to find Luigi in the background. Peel him off by Paperizing from all five locations to unlock him in the credits. Each time you peel off Luigi, you can read a related newspaper article in the leftmost house in Decalburg's Residential Area.
World 1-6: On the bridge leading to the central tower.
World 2-5: On the fourth floor of the tower; in a window at the far right end of the second outdoor walkway.
World 3-12: On a rock formation sticking out of the ocean; only visible from the pier.
World 4-5: Near the end of the chairlift after the enemies stop coming; appears only briefly.
World 5-5: In the background to the left of the hot springs.