Let's start with some self-made general descriptions of both phases of this forme, with the minor design changes not mentioned:
1. Greninja coats itself in a spinning water veil to protect itself in battle. Asides from that, not much has changed. (only lasted a few episodes, it's basically spoiler-preventing)
2. Greninja coats itself in a water veil, which slowly turns into a large Water Shuriken on its back. This improves its speed and the shuriken can be used to throw many Water Shurikens as possible in a single battle. Also, it shares its heartbeat with its Trainer. (the "true" forme)
How would Simcha Whitehill, who writes the majority of them nowadays interpret both? He stated “Ash’s amazing Greninja” on page 234 of the Visual Companion ("second" (read: third) edition) but no other straight mentions asides from some promo art of Ash-Greninja, fittingly placed under Ash's own Greninja while other ones would have a regular Greninja. I'd love to hear what she'd believes those formes are meant to be described as, sort of...
1. Greninja coats itself in a spinning water veil to protect itself in battle. Asides from that, not much has changed. (only lasted a few episodes, it's basically spoiler-preventing)
2. Greninja coats itself in a water veil, which slowly turns into a large Water Shuriken on its back. This improves its speed and the shuriken can be used to throw many Water Shurikens as possible in a single battle. Also, it shares its heartbeat with its Trainer. (the "true" forme)
How would Simcha Whitehill, who writes the majority of them nowadays interpret both? He stated “Ash’s amazing Greninja” on page 234 of the Visual Companion ("second" (read: third) edition) but no other straight mentions asides from some promo art of Ash-Greninja, fittingly placed under Ash's own Greninja while other ones would have a regular Greninja. I'd love to hear what she'd believes those formes are meant to be described as, sort of...
