Nurses appear in the first two games of the series but are absent in Kid Icarus: Uprising, where their services are instead performed by Palutena. Nurses run Hospitals, of which there is at least one in every Fortress. They exist primarily to rid Pit of the Eggplant Curse brought on by the Eggplant Wizard. In fact, they only appear to rid Pit of the curse; if Pit does not have the curse, they will not appear.
Nurses appear in this game exclusively within Fortresses. A common misconception often leads a naive Pit to believe the nurse will heal him health-wise, though when she does appear, her speech shows otherwise. Pit probably wouldn't care if she healed him or not, though, because simply being able to attack again is more than enough relief, due to the frustration caused by the curse.
Nurses appear in this game as well. They serve the same purpose of removing the Eggplant Curse as in the original game.