Green Key

The Green Key is an item found in Mario & Luigi: Bowser's Inside Story. When Bowser reaches a spot with several doors on the second floor of Peach's Castle, he will find Fawfulcopters holding keys. Bowser can optionally chase the one holding the green key. When the player does this, the Fawfulcopter will eventually be cornered to the southeastern corner of a room filled with spikes. With the green key, Bowser will be able to open the green door in the original spot and get the Economy Ring.