It is a shame that the Japanese promos are increasingly going the way of the English event promos where everything is just a stamp variant and nothing is overly unique. While that isn’t a rule or anything, each new one makes me a bit nervous that we will lose more unique card opportunities to lazy stamped cards.
As far as I know, it’s supposed to be more of a psuedo-Mega form for Ash’s Greninja. It doesn’t have the unique look all the time and reverts back to a regular Greninja when not in battle. Basically instead of giving Ash an actual Mega, they decided to do Ash-Greninja… I still think it would’ve been better to give him a Charizard Y or something as two different Charizard X have been featured in anime already (Red’s in Pokemon Origin and Alain’s in the Mega Evolution specials).