Red Key (Mario & Luigi: Bowser's Inside Story)
From the Super Mario Wiki
The Red Key is an item found in Mario & Luigi: Bowser's Inside Story. It can be found on the second floor in Peach's Castle. It is carried by a Fawfulcopter which Bowser chases after. When Bowser chases it to the highest spot on the cliff using the Spike Ball ability, the Fawfulcopter has nowhere to go and Bowser can hit it for the key. With the red key, Bowser will be able to open the red door he encountered earlier and save his Magikoopa minions, thereby getting the Magikoopa Mob attack.
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