Pencil

Pencils mark the chambers that Pit has entered in a Fortress. When used in conjunction with a Check Sheet and Torch, Pit can track his location easily. The Pencil is discarded when a Fortress is cleared.

Kid Icarus
Pencils can be purchased for 180 Hearts in a Fortress Store. An intimidated shopkeeper won't sell them for less, but an angered merchant will raise the price to 250.

Kid Icarus: Of Myths and Monsters
Pencils are 150 Hearts here, but can also be found inside Centurion statues.