Ash's Greninja (Aura)

Ash's Greninja was the first Pokémon that Ash Calem Ketchum obtained in the Kalos region.

Personality and characteristics
As a Froakie, he was a stubborn, picky Pokémon, displayed by the fact that it had several other Trainers that he refused to listen to, either forcing them to return him to Professor Sycamore or abandoning them and returning to the lab itself. He is also extremely similar to Ash personality wise, as he is not afraid to leap into danger regardless of its own safety and doesn't back down from a fight. It is clear that he recognizes these traits in Ash, hence why he was finally willing to accept being taken into Ash's wing. He also has a sense of honor, as demonstrated when he attacked Fletchling just because it made Mia cry. He is shown to have a good judgement of character. Froakie is prone to blushing when praised, as shown when being thanked by Mia and Fennekin. Greninja as a Froakie gets embarrassed when losing in a bad way, such as when he lost because of Attract.

Since evolving into a Frogadier, his personality remains largely the same. He is still shown to be loyal to Ash, as shown when he hugged him after his victory in the Coumarine City Gym battle. Greninja is a responsible Pokémon that can take initiative to protect friends and allies. Greninja also appears to be good with younger Pokémon and children. Greninja also seems to care deeply for its friends, as seen when it acted as bodyguard for Serena and many others, and protected the two from a wild Pangoro.

Greninja seems to appreciate and respect bravery and selflessness, and as such, aims to be so himself. He respects those with these traits, as he does Ash. Greninja was impressed and surprised by Ash's willingness to put himself in danger to help a Garchomp that wasn't his Pokémon and barely knew, and admired this show of self sacrifice and selflessness to the point Froakie went alongside Ash to face Garchomp. He also showed genuine concern for Ash's safety when Ash jumped off the Prism Tower. A lack of these traits in his previous Trainers may be what caused Greninja to leave them.

Greninja has been shown to enjoy battling, and seems to enjoy the opportunity to fight a strong opponent. Once recognizing Sawyer's Sceptile's strength, he developed a rivalry with it. He also seems to relish at both the thought, and the action of defeating a strong opponent, as seen when he grinned before fighting Sawyer's Sceptile, and defeating Grant's Onix.

Ash-Greninja
Shortly after evolving into Greninja, through his strong bond with Ash, he can call upon a transformation similar to Mega Evolution, where he takes on an appearance resembling his Trainer while being covered in a water veil. In its final form, the water veil moves from around his body to his back and forms an "X" shape, which resemble wings, then the wings form into a small sphere shape of water on the middle of his back with a light blue ring in the middle of it and it then grows in size until it forms into a giant shuriken made of water. Its full capabilities are unknown, but it appears to give a significant boost in speed and power. This form comes with a side effect where whenever Greninja takes damage, the pain is shared with Ash in the same location where Greninja was hurt. Ash is also shown to be able to see from Greninja's point of view and will occasionally mimic Greninja's movements. Ash describes it as feeling like he "became" Greninja. It is revealed through a device Jacob created that Ash and Greninja share the exact same heartbeat while Greninja is in the form. If the form is pushed too hard, it's possible for Ash to pass out from the resulting exhaustion and pain. As such, Mia coined the term "Ash-Greninja".