Complete all the missions from the indicated character at the apartment building. That character will offer to come over to Kirby's Pad after you complete a level. Return to Kirby's Pad and that character will give you a bonus wallpaper.

Beadrix Print: Complete Beadrix's missions and have Beadrix visit Kirby's Pad.
Buster Print: Complete Buster's missions and have Buster visit Kirby's Pad.
Carrie Print: Complete Carrie's missions and have Carrie visit Kirby's Pad.
Mara Print: Complete Mara's missions and have Mara visit Kirby's Pad.
Zeke Print: Complete Zeke's missions and have Zeke visit Kirby's Pad.