Doctor (Doctor Who)
As a Gallifreyan the Doctor looks like a human on the surface, but there are differences. One well-known difference is that The doctor has two hearts. As a Time Lord the Doctor can also regenerate, if doctor whos body is badly hurt and the doctor dies. During the regeneration, the body is healed and renewed, which includes a different appearance and personality. The regenerated person is the same person, with the same memories and basic personality traits; but the person also looks different and a changed personality. Because of this the Doctor could be played by different actors. This made it possible for the series to be filmed over a time of nearly fifty years without having to actually "change" the character.