I was pretty shocked they were including this alternate form into the games. The games haven't really had much link with the anime series itself outside of Pokémon Yellow which borrowed a few aspects (Pikachu following you, getting all the starters throughout your journey, Team Rocket trio etc) but it seems like these games have made real nods.
Not only is it hinted the special Greninja comes from Ash but when you're in the abandoned megamart snapping up pictures in that final room, there's one which looks suspiciously like Ash + Pikachu.
All in all, not a bad form. Once it gains momentum, it's pretty devastating. Not one i'll be using though, quite a few other new toys to be playing with.