I'm sorry but I can not support this backstory, Greninja's (Frogadier at the time I know) backstory while sad feels largely self-inflicted. It was ostracized by its fellow Froakie but only because it refused to socialized and was only interested in training, and on top of that it was way too picky with who it wanted for a trainer.
Compared to Charizard, Infernape, and Pignite who were all abused, and in the case of Charizard and Pignite put into life threatening situations, Greninja's seems tame in comparisons since like I said it just feels self-inflicted