Nancy is the nurse that works with the doctor in the clinic in Harvest Moon GBC 2.
She arrives in the Spring of the second year to assist the Doctor and works at the clinic. She and the doctor will assess your character's energy levels and health. If you are sick, then you will need to spend the remainder of the day and night at the clinic, and you will be released the next morning.
She and the Doctor have an event where the player becomes almost deaf if the player plays past the third year.