Key items are essential for progressing through the main story and unlocking new areas. These include various keys (e.g., Iron Insignia Key, Dimitrescu's Key), crests, and puzzle pieces (e.g., the Four Masks, the Beneviento Doll parts). Each key item has a specific use and is often found after solving a puzzle or defeating a mini-boss. The game's inventory system clearly marks key items, so you won't accidentally sell them. Pay attention to their descriptions for clues on where to use them.
