>>22070395Because, unlike what your opinion of it is implying, it's one of many great designs in the pokemon series. The fact that it's a starter helps it to be easily remembered, add to that the exceptionally unique design quirk. Because, seriously, if you think Tonguescarf, Greninja is guaranteed the only result. Distinguishing aspects like these make for memorable design. It helps that Greninja is overall well designed.
Oh, and apparently the guys at Bandai Namco and Sora thought similarly, seeing that they unanimously decided onto adding him into Smash. AKA: Those two pokemon are the only post-Gen I things that i know about.
And do i seriously need to mention its competitive usefulness?
It hit all the right marks. Asking how is it popular is honestly a pretty dumb question.
