Ash's Greninja was evolved from a Froakie that is the first Pokemon Ash caught in the Kalos region in one of Pokemon anime series, Pocket Monsters: XY & Z.
New ability "Battle Bond"
The Graninja has a new ability, called Battle Bond. Once the ability is triggered by beating a Pokemon, it allows the Pokemon to change a form of itself only during a battle.
How to get Ash's Greninja
In special demo version
You can use Ash's Greninja in Pokemon Sun and Moon by playing the special demo version of the game that was released on October 18. In the demo, a certain Pokemon trainer entrust the Pokemon with you. After being transferred from the demo to your game data, it becomes available.
Bast stats of Ash's Greninja described in the table above is that before Battle Bond is activated, and IVs and EVs are determined. For used acronyms, see the table below.