Is there some previous game she was in that established her popularity, or is it more about current FEH players liking dancer and singers, or about current FEH players liking her character design and/or voice acting?
It could be a combination of all this but it’s most likely due to fans of FE7. Ninian plays a big role in the plot and she definitely developed really well through the game as a character so I’m not surprised to see so many people want to support her after the events of FE7 and how popular that game specifically is in the community
Blazing Blade was the first game to see international release meaning Ninian was the first dancer many players were exposed to. So nostalgia is a factor. She's also generally considered Roy's mother though it's not actually canon (just highly favored in the game) and, as the CYL demonstrated, Roy is the second most popular male Fire Emblem character (thank you Smash Brothers).
Also... Not to be negative but consider the competition...
M!Corrin/F!Corrin are probably the most popular characters up there but both suffer from overexposure. I have nothing against them but I'm tired of seeing them all the time and I suspect that others feel the same.
Y!Tiki is very popular in Japan but Shadow Dragon is the only game featuring her to see international release. I liked Shadow Dragon personally but it's very unpopular to the point that it almost marked the end of the series.
A!Tiki is a side character - basically an easter egg - in Awakening who doesn't really get much in the way of character development.
Fae and Sophia are featured in games that never saw international release. That immediately hurts their recognition.
Nowi surprised me... I'm not a fan of her myself but I thought she'd be more popular.
I took screenshots of the initial votes an hour after the gauntlet started, and Nowi was the third most popular, just a few thousand behind young Tiki. This gauntlet has been more about manipulating the phases than raw numbers. With everyone aside from Ninian, that is, haha. She's pretty much unstoppable atm.
It was a surprise for me, too. I'm not at all against it. If Tiki loses against Corrin, I'll probably join her, not that I have anything against either Corrin. I just really like her design and she seems really sweet and kind of melancholy.
Obvs I've never played 7, and I don't really intend to, though anything could happen. But this game is really good at getting me interested in earlier characters, and maybe that was the entire point. I'm happy she's doing well, even if her being basically a runaway train in this gauntlet is kind of scary.
Yeah, FE7 is pretty popular among international audiences seeing as that's the first game many of us played in the FE franchise. Yet I don't remember Ninian being particularly popular as a character back then, despite her indeed being the closest thing to a "canon" parent of Roy (you can marry off whoever you wanted to of the designated support options, but romance between Eliwood x Ninian and to a much lesser extent Hector x Lyn seemed to be implied).
I do think a big part of it has to do with the fact that Ninian is a dancer (so naturally S tier) who has been featured in a few banners, so everyone has her. People want to join the team of a unit they actually have for those bonus points, since that's all that matters since there's no real character-based incentive. Several of the characters she's up against are either niche, or haven't been featured in banners in ages, if ever, like Sophia, Fae, MCorrin, FCorrin (Awakening/Fates lord status pretty much overrides how crappy a unit is in battle, or in terms of personality). I remember building Sophia back when she was borderline considered a meme, and now with the focus I'm JUST beginning to see some of her in arena.
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