The principle advanced by some parapsychologist that there are thousands of ghost that haunt the world's large cities because that is where the most people live and when they die some of them become earthbound and haunt the cities. And over the years those cities acumulate more and more ghost Also many ghost like to be around where there are a lot of people because they like the energy that comes off them and they can use that energy to remain earthbound.
So when you see a person on a street corner in a large city it is probably just another person but there is a chance it is not.

Someone people turn to when they're sad who makes them feel better.