Keys are items that appear in two games of the Kid Icarus series. They can be used to open certain doors or re-enter rooms that have already been accessed before.
Kid Icarus: Of Myths and Monsters
In this game, keys allow Pit to re-enter a room once he has left it. A key can be purchased for 300 Hearts, or 250 if he barters with the merchant (failure to intimidate doubles this price). The Black Marketeer sells this for 400. Collecting the Palutena's Key, which only appears once per stage, opens all doors.
Kid Icarus: Uprising
Keys reappear as a minor item in this game. In Chapter 3, Heads of the Hewdraw, a key is needed to enter a courtyard where Pit will face off with a Hewdraw Head. In addition, another locked door can also be seen guarded by the Eggplant Wizard in one of the game trailers. Palutena's Key also makes an appearance in Chapter 20, Palutena's Temple.