Ditto is a free-form shape blob with vestigial facial features. Ditto is a normal type pokemon which can use the move transform giving it the ability to transform into anything.
Ditto's primary defense mechanism is its ability to rearrange its cellular structure. This permits it to transform into essentially any other physical object. When two Ditto meet in the wild, they will attempt to transform into each other. This tendency makes Ditto a unique Pokémon in battle, as it will adapt the form and abilities of its foe.
Ditto is best able to transform into something if it has a model to work from. Although Ditto can transform via memory, it has a tendency to forget important details, particularly with its foe's face. Ditto will also transform into a rock when sleeping to avoid attack.
Ditto has appeared in the anime under the ownership of a trainer called Duplica, Duplica's Ditto at first had a problem when using its move transform but was able too master this technique after being kidnapped by Team Rocket.