Ash's Greninja (Forever)

Ash's Greninja is the first Pokémon that Ash obtained from the Kalos Region.

At Professor Sycamore's Lab
TBA

With Patrick
TBA

Personality
Greninja seems to appreciate and respect bravery and selflessness, and as such, aims to be so itself. This is first shown when it took a hit for Pikachu during Team Rocket's attack despite not knowing Pikachu or his Trainer, and then proceeded to help them both in combat. It respects those with these traits, as it 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 Frogadier went alongside Ash to face Garchomp. It also showed genuine concern for Ash's safety when Ash jumped off the Prism Tower. A lack of these traits in its 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 Grovyle's strength, it developed a rivalry with it. It also seems to relish at both the thought, and the action of defeating a strong opponent, as seen when it grinned before fighting Sawyer's Grovyle, and defeating Grant's Onix.

Greninja's close bond with Ash was also one of his greatest insecurities, as their repeated failure to master the Ash-Greninja forme started to weigh heavily on trainer and Pokémon alike. When Ash left Greninja in the Pokémon center after the loss to Wulfric, out of a sense of guilt for Greninja's injuries, a saddened Greninja gave chase as soon as he recovered. It was revealed Greninja also felt responsible for their loss. After fully synchronizing their feelings to save a Spewpa, however, Ash explained to Greninja why he wanted to become a Pokémon Trainer, and that he saw his Pokémon as his closest friends. After that, Greninja and Ash came to fully understand each other.

Ash-Greninja
Shortly after evolving into Greninja, through its strong bond with Ash, he can call upon a transformation similar to Mega Evolution, where it takes on an appearance resembling its Trainer while being covered in a water veil. In its final form, the water veil moves from around its body to its back and forms an "X" shape, which resemble wings, then the wings form into a small sphere shape of water on the middle of its 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 were unknown, but it appears to give a significant boost in speed and power. In its completed form, it can rival the power of Mega Evolved Pokémon. 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 by Clemont's device that Ash and Greninja share the exact same heartbeat while Greninja is in the form and that the form can be accessed when Ash and Greninja's feelings synchronize. If the form is pushed too hard, it's possible for Ash to pass out from the resulting exhaustion and pain. The term "Ash-Greninja" was officially coined by Bonnie. During the next episode, Ash and Greninja made amends and were able to trigger the transformation at will. By the time of their rematch against Wulfric, Ash and Greninja were able to use the form without passing out, and although they still shared each others pain, it seemed to be more manageable for Ash to handle.

Trivia
Greninja is Ash's first Pokémon to be owned by more than one Trainer before him.
 * With Froakie, this is the first time that Ash catches a starter Pokémon as his first Pokémon in a region.
 * Greninja is also the first of Ash's first regional catches that is not a Normal/Flying dual-type since Heracross, as well as the first one that is incapable of flying.
 * Greninja is the first Water-type starter Pokémon to have ever evolved under Ash's ownership.
 * Greninja is also the first fully evolved Water-type starter Pokémon owned by a main character.
 * Greninja is also Ash's first Water-type to have evolved twice.
 * Greninja is the first of Ash's starter Pokémon to have its Egg revealed on-screen.
 * It is also the first Pokémon owned by a main character in the anime to have evolved twice after hatching from an Egg.
 * Greninja is Ash's only Kalos Pokémon that hasn't been weak to Ice moves at any point.
 * Ash's Greninja is the only Pokémon that possesses a new evolution technique which hasn't been known yet.