There is no pokemon which evolves that does not lose some part of itself through the process of evolution. And even species which do not undergo such a dramatic metamorphosis, such as humans and non-evolving pokemon, often find that they left something important behind in exchange for strength and maturity. In no species, however, is this transformation nearly as painful as in the Nincada, who are split in two; an extremely fast and fairly powerful bug named Ninjask, and a fragile spirit of childhood called Shedinja.
It is said that Shedinja steal spirits, but this is a poor understanding of the personality changes they can create. People who stare into the backs of Shedinja change dramatically, but it is less a loss than a reversion; that part of their personality they lost comes to the forefront. It does not take the form of a Charizard saying Charmander or anything of that sort, but of an idealistic person or pokemon remembering what they once loved and following their forgotten dreams.
Yet to the world they live in, such idealism and passion is seen as only fit for children. Perhaps this is why so many children come out of nowhere to become Pokemon League Champion. Shedinja may not be much use in a battle, but there is much that can be learned from them all the same; they do not steal spirits, but restore them to their proper form.