Ka are spirits or the vital essense of a person that remain on Earth. On Earth, they are often used as Ka servants that link the world to that of the Spirit World. According to the spirit's curse, they are required to stay on Earth whether they like it or not. They have special abilities such as being able to tell if a person is dead and special knowlege of the spirit world.
They were originally created by the Egyptians to gently explain to other spirits what had happened tot hem after they had died and bring them to the spirit world.