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Is he actually popular with kids, or did GameFreak just keep pushing him in hopes he would be popular with kids?
There's a shit ton of kids at a tutoring center I work at that love Pokemon and I know they get hyped for Mewtwo + his mega forms and Charizard, but I've never heard any of the kids get hyped about Greninja. This is purely anecdotal, obviously, but it's all I've got to go by.