There is a certain pain associated with prolonged loneliness, a suffering which can drive humans and pokemon mad, and this bitterness does not always cease with death. Mimikyu resent their status as ghosts, finding that they pass with even less notice than when they lived, and frequently resort to poltergeist antics in the hope of any social contact. Worse, some Mimikyu grow so desperately lonely that they will lift their disguise and absorb the souls of the living, as they know of no other way to be kept company. It is widely held that the custom of funerals developed to remind the spirit of the departed that others cared about them in life, lest it linger in this world as a Mimikyu.
Yet loneliness comes with solitude, and solitude comes with the time to create amazing things. Mimikyu's costumes are a disguise the envy of contest trainers, although they often lack sufficient materials to imitate a healthy Pikachu. Some even keep up the illusion in battles, for Mimikyu, in their long afterlives, have become expert copiers of all sorts of pokemon attacks – among them Pikachu mainstays such as Thunderbolt. But the longer any fight lasts, the clearer it becomes that Mimikyu have overshot their mark; their disguise is far more resilient than a Pikachu's fragile rodent body, and Mimikyu continue fighting long after their "bodies" receive apparently fatal wounds.
It is not unheard of for Mimikyu to battle against a trainer who recognizes its potential and invites it to join their team, or even to battle with such a trainer in the audience. When this happens, Mimikyu gain lifelong friends and teammates, and their loneliness is vanquished; they linger in this world to help their trainers fulfill their dreams, and at last depart it together with them.