Key

Keys are items that debuted in Kid Icarus: Of Myths and Monsters. Appearing in Solo Mode in Kid Icarus: Uprising, keys grant the user access to locked doors.

Kid Icarus: Of Myths and Monsters
Allowing Pit to re-enter a room he left, keys can be purchased for 300 Hearts, or 400 Hearts if purchased through the Black Marketeer.

Palutena's Key
Palutena's Key only appears if all rooms in a single stage have been visited and are now closed. It will be hidden in a block near the end of each stage, which must be smashed open with a hammer. Once obtained, the key re-opens all locked doors within the stage.

Kid Icarus: Uprising
In Chapter 3, a key is needed to enter the courtyard where the Hewdraw Head is located.

In Chapter 20, a Palutena-themed key is needed to proceed through Palutena's Temple.