Rosalina should not be considered less worthy than other characters in the Mario franchise just because she has only had a major role in 2 games (Galaxy 1 and 3D World). Her role in Galaxy 1 was what made her popular, and it was what made her special. Her one appearance in Galaxy 1 made her much more respectable, dignified, and just flat-out interesting and layered than Daisy, Waluigi, Toad, and Bowser Jr (who could have been much more but they dropped the ball with him after Sunshine) could ever hope to have been in the array of games they've appeared in, and her personality, development, role, and backstory are some of the most innovative things the Mario franchise has had going for itself. The fact that she has become a mainstay of the series is irrelevant. Her one appearance in Galaxy 1 was always enough justification for her to be in Smash, and I've thought so since the Brawl days. And if you want to tell me I'm wrong, talk to the Palutena supporters, or supporters of any Kid Icarus character. Uprising is the reason anyone wants Palutena or any Kid Icarus character. Nobody was asking for Palutena or Medusa before Uprising, and Hades and Magnus only appeared in Uprising, yet people want those guys too. Also, talk to the Wonderful 101 fandom. It's a franchise with one game, yet a lot of people wanted to see a character from that in the game.
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This is tangentially related, but Rosalina's Smash appearance partly spawned this - where is this "Rosalina is Nintendo's Lightning" idea coming from?
For those of you who don't have a clue what I'm talking about, people seem to have this idea that "Lighting from FFXIII is in everything Square Enix makes". And due to Rosalina showing up in a few more games lately, people think she is "Nintendo's Lightning".
Firstly, Lightning has only had four note worthy appearances (the 3 FFXIII games, and Dissidia: 012. In FFXIII-2, she actually was only a supporting character. Her role and presence was about as significant as Rosalina's in Galaxy 2). Any appearance Lightning has outside of that has been as an EXTREMELY minor cameo, ones that most people aren't even aware of. Heck, some of these games are Japan-exclusive, and they all include cameos from every FF character under the sun anyway, so her presence is hardly special. Plus, Lightning is considered a significant role model in Japanese culture, so it makes sense that she is promoted in Japan anyway. Lightning's been heavily promoted there, but in terms of well known games, she's only had four. Most of her promotion has been in other media. And she's not the only character to get this treatment, all of the most popular FF characters do.
Rosalina is literally the exact opposite. She's become a mainstay of the Mario series, so she shows up in most games the other big mainstays do. And she's in Smash now. Outside of that? She's had little to no promotion. There are only 3 pieces of official Rosalina merchandise you can buy, as far as I'm aware (two of which I'm not 100% sure are official). She's been in this obscure Mario manga too, but only briefly. And her only note worthy roles are in the Galaxy games, 3D World, and in the future, Smash Bros. Every other title is just games that feature every other Mario character anyway.
While I can draw a lot of parallels between Lightning and Rosalina, I definitely don't see their promotion levels as equal at all, and Lightning is not really a shoehorned character to begin with, and Rosalina certainly is not.
At this point, I feel like people just need someone to blame for other characters not being in Mario games and Smash, and they rationalise that Rosalina prevented those characters from making it in, for whatever reason.