You may be looking for the inhabitants of the Shadow World.

The Shadow People were an alien race created by the Shedagen.

The Shedagen needed the Shadow People to serve them. They gave them empathic powers, so that the Shadow People were able to find out what the Shedagen needed. To serve all of the Shedagen correctly, the Shadow People had to become them. They got the appearance, personality, thoughts and memories of the Shedagen. Through this they got to know what the Shedagen wanted. Because the Shadow People only existed to serve others, they stop existing when every other creature around them has either left or died.

They always imitated the appearance of other creature around them. However they had red eyes, when they were angry.

Sarah Jane Smith met one of the the Shadow People. It travelled with the Shedagen in space, when the spaceship crashed on Earth. The Shedagen lived the rest of their lives on earth. The Shadow Person served them until their death and made sure that they died happily. When the last one of the Shedagen died it also stopped existing, because there wasn't anyone to serve. Years later Rani Chandra's, Clyde Langer's and Luke Smith's class got there during their school trip and the Shadow Person started to exist again. However, it hadn't existed for so long that it did not know who it was. It thought it was one of the students and it imitated the appearance of Rani Chandra, Clyde Langer and Luke Smith. Later its memories came back. However, it also knew that it would stop existing as soon as the class left. It wanted the class to stay so that it could serve the class. However, Sarah Jane convinced it to let the class leave and be itself even if that meant that it would stop existing. It waved Sarah Jane goodbye until it finally disappeared. (AUDIO: The Shadow People)

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