There is a secret set of wallpaper that cannot be purchased at the fabric store. After completing all of the missions from a character in the apartment building, that character will offer to come over to Kirby's Pad after you complete a stage. Go back and visit Kirby's Pad and the character will be there, just as if they were visiting normally. However, after making their standard introduction, the character will present Kirby with a special wallpaper print. If you miss the character (for example, by going into a level before visiting Kirby's Pad), simply complete another few stages and the character will once again offer to visit Kirby's Pad.