Key items are unique, non-consumable items essential for story progression or unlocking new areas. These include items like the Paper Macho Goomba Mask, the Origami Castle Key, or various Vellumental powers. You'll acquire key items by completing objectives, solving puzzles, or defeating bosses. Collectibles, such as Trophies and Toads, are optional but contribute to game completion and often unlock rewards. While not directly used in battle, key items are vital for navigating the world and advancing the narrative. Always check your inventory for key items if you're stuck on a puzzle or can't progress in an area.
