Nurse Joy is a recurring character in every Pokemon series released.
There are multiple Nurse Joys in every town, city and region. They all look the same, have the same voice and personalities. There have been numerous speculations as to why all the Joys look alike, though it seems they are all related in some way.
However, there have been instances in which a Nurse Joy has been known to have a different personality and appearance (such as the aggressive sun-tanned Nurse Joy of the Orange Islands). Brock has an ability to tell each Joy apart based on minute details in their differences.
Each Joy works in the field of medicine, loves Pokemon and is extremely patient and caring. They are the heads of their respective Pokemon Centres.
A Nurse Joy’s Pokemon choices are predominantly Chansey and Blissey: nurturing Pokemon that aid in the Pokemon Centres.