Make that 20 - for some reason I included Greninja and Lucario in the original list.
Mario, Link, Kirby, Ness, Ice Climbers, Dr. Mario, Young Link, GNW, Pokemon Trainer, Olimar, Villager, Wii Fit, Pac-Man, Robin, R.O.B., Joker, Hero, Byleth, Steve
Again, you could make arguments for and against many of them. Like, Olimar has a notebook that gives away his personality and he also crashes his rocket ship and can be very clumsy, so I can understand in that case. Or like Robin or Byleth or Hero or Joker (I've never played those games so IDK but they're avatar characters in an RPG so I just assumed). Still, it's a non-zero number, which is the point I was trying to make.
It's an even smaller number than this.
Mario himself is absolutely not devoid of personality like the original claim alluded to; his personality is fairly well defined. It's
standard, yes, but it's there. He's neither silent nor a blank slate.
Olimar is also one of Nintendo's most fleshed out protagonists. He's neither silent nor a blank slate; some might even argue he's among the most fleshed out leads Nintendo has, especially for one that isn't from an RPG.
Robin is absolutely not a silent protagonist without personality, either. In fact, aside from gender, hair color, name, and build, Robin is very much a defined and unique character with a set story. They're honestly an avatar in name only compared to the likes of Steve or Villager.
Pac-Man also has a set personality and isn't silent, particularly in Pac-Man World 3 where he's very snarky and sarcastic.
Smash itself takes Subspace as canon when pertaining to R.O.B., so I'd also not call him a blank slate, silent as he may be. Similarly, Smash's portrayal of Wii Fit Trainer also plays up the fitness nut aspect of her. These two aren't much, but it's important to recognize that it's
significantly more than Piranha Plant has.
That honestly goes for Joker, Hero, and Byleth as well. They're more traditional silent protagonists, yes, but they're also not devoid of personality in their home games, between Joker's possible dialogue choices, Byleth's stoicism and in-universe justification for it, and Hero having quite a few moments of levity in DQXI.
That's all to say that trying to gas up Piranha Plant as having "the same amount of personality as any other Nintendo character on the roster" is ludicrous. On the Nintendo side, quite literally the only other ones as devoid of a concrete personality are either ones that are Literally Animals (Pikachu, Pichu, Jigglypuff, Squirtle, Ivysaur, Charizard, Duck Hunt), or are
actual blank slate avatars (Villager and Mr. Game & Watch...Steve would be here too if we counted third parties). And even
then, most of these exhibit more of a personality than Plant does, particularly the Animal group.