Black fins at the sides of the head, resembling Ash's hair
Body of a lighter shade of blue
A large water shuriken on its back (Ash-Greninja throws multiple of those shurikens at its foes)
Often seen with a straight posture and standing on one leg
Usable in the Generation 7 games via a special Greninja obtainable from the Pokemon Sun and Moon special demo that could be transferred to the full game. This Greninja had a fixed ability called Battle Bond: when it knocked out a foe and didn't win the battle from that KO, it would transform into Ash-Greninja which drastically strengthened Greninja's attacking stats and gave a slight boost to speed and made the move Water Shuriken always hit 3 times in those games.
The form was retired when Greninja was made usable again in Pokemon Scarlet And Violet with a massive change to the Battle Bond ability that dropped the previous benefits and the transformation in favor of just a one time stat boost to attacking stats and speed when making it's first battle KO likely to coincide with the fact Ash-greninja status as a anime tie in to a character no longer in the anime with the retirement of Ash has the protagonist from the anime has of Pokemon_Horizons.
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