Mimikyu is a small Pokémon whose body is almost entirely hidden under an old rag. Its small size makes it one of the shortest Ghost types. Its beady black eyes are visible through holes in the body of its disguise, and the fringe of an amorphous foot or lower body is visible under the hem. Occasionally, it will also extend a black appendage from under its rag. The veil itself resembles a Pikachu. The cloth is yellow with black-tipped ears. The Pikachu face appears to be drawn with crayons, consisting of black eyes, red cheeks, and a squiggly black mouth. The head is hollow and can be used to store objects. Mimikyu also carries a small stick resembling a lightning bolt to create a tail for its costume. When its disguise is damaged, the costume's head droops limply backward, revealing Mimikyu's ruse. Larger rips in the costume are accompanied by shadowy vapors pouring out from whatever is hidden within.

A lonely Pokémon, Mimikyu is always covered by its disguise. It is unknown what Mimikyu's true appearance looks like. Since this Pokémon is weakened by sunlight, it is rumored that the veil is used for protection. It is believed that seeing its true form will cause a mysterious illness or even a painful death. Because of this, Mimikyu will become agitated and violently stop anyone from attempting to look underneath its rag. Mimikyu's disguise is modeled after Pikachu because of the popularity of Pikachu-styled merchandise, thinking that its disguise would allow it to make friends with people. Unfortunately, this attempt makes its disguise look more creepy. Until recent times, Mimikyu was initially thought by humans to be just a ghost wearing a cloth. Mimikyu is the only known Pokémon capable of using the Z-Move Let's Snuggle Forever. It is also the only known Pokémon that can have Disguise as an Ability.

A special Mimikyu in Alola serves as the Totem Pokémon for the Thrifty Megamart (Abandoned Site)'s Island trial.

Forms

Mimikyu has two forms: Disguised Form and Busted Form.

Mimikyu changes into its Busted Form when it is hit by an attack and its neck breaks, causing it to slump over and no longer convincingly resemble a Pikachu. Ordinarily, the rag it wears allows Mimikyu to avoid attacks. Mimikyu will attempt to keep up its disguise, even if its rag is broken. However, if its rag does get damaged or its neck gets broken, it will spend all night patching it in front of a mirror as if its life depends on it. Repairing the rag correctly is often difficult for Mimikyu, so it cries a lot on the inside. It then seeks revenge on the one that damaged it, fearing that its true identity could have been exposed. Unforgiving, Mimikyu will take down the culprit at the risk of costing its own life in the process. Mimikyu will never forget the person who damaged its rag for as long as it is still alive.

(Source: Bulbapedia)