Ash’s special Greninja transformation in the Pokémon TV series is called Ash-Greninja. It appears when Ash and Greninja form an unusually strong bond during the Kalos storyline.
The transformation is associated with the Bond Phenomenon, a rare connection that changes Greninja’s appearance to resemble Ash. Their shared sensations and coordinated movements make the form different from an ordinary evolution or standard battle technique.
Ash-Greninja becomes central to Ash’s battles in Pokémon the Series: XYZ, including major Gym and Pokémon League matches. The form is later connected to the Battle Bond Ability in the Pokémon games.
It is often incorrectly called Mega Greninja because its power and appearance resemble Mega Evolution. However, Ash-Greninja is a distinct transformation and is not a Mega Evolution.