The idea that all sword characters simply do well/beat Rosalina needs to go out of the window.
The only characters with swords that
legitimately beat/have an advantage over Rosalina are Meta Knight, Cloud, and Corrin; these characters have tools to not only deal with Luma, but effectively deal with Rosalina.
This is a big deal that others lack. Meta Knight is self explanatory and well established. Cloud is arguably a less oppressive (but still oppressive, if you understand me) version of Meta Knight in the tools he has at his disposal and what they do to Rosalina. The reason Corrin does good against Rosalina (as opposed to Marth or Lucina, as you are all probably wondering), is the fact that Corrin does what Marth and Lucina want to do, with fewer button presses, while possessing less of their disadvantages in the MU, which are overlooked and will be explained below.
Marth and Lucina do well against Rosalina in the neutral, provided Rosalina is playing her more defensive style. If however, Rosalina chooses to play the more agressive neutral game (in fact, only backed up by Kirihara against Komo's Marth yesterday), she has many tools to compete with them; Dash Attack, Nair, RAR Bair, Dtilt. All of these moves allow Rosalina to outspace them and create openings. Now some of you will be saying "but their sword can deal with Luma effectively". This is true in a sense... but what are they going to do when Luma is gone, huh? Go back and watch Komo's Marth game with Kirihara. There's not a huge deal Marth can do against her. Of note, Dash Attack clanks with all of Marth and Lucina's grounded attacks, which is also a boon. Another big difference in this MU are the ways each are able to capitalise off of an opening. Marth ans Lucina's actual combo game is relatively limited; the amount of actual combos they can get on Rosalina is not very extensive due to her lower weight and fall speed often launching her too far; Dthrow combos are more or less useless against her. Rosalina can capitalise off of a hit much better than they can. All this is before considering that Marth and Lucina struggle to land;
THIS IS THE MOST IMPORTANT PART OF THIS MU AND IS OVERLOOKED ON MANY AN OCCASION. Rosalina's BEST trait is arguably her ability to juggle with Uair. Marth and Lucina have landing issues. This is a reason why Corrin does better in the MU, as Corrin does not get juggled as easily as them, as Dair is able to challenge Rosalina's juggle attempts thanks to it's stall then fall property and forgiving hitboxes (as well as being good at getting rid of Luma, even if Rosalina shields the landing hit, it can get rid of Luma. Marth and Lucina deal with Rosalina better than most characters, but they do not straight up have the advantage merely because they possess a sword. This MU is much more even than the average player would think (please note that I main Rosalina and Marth so I can see both sides of this MU).
Characters such as Ike and Roy inherit the same issues as Marth and Lucina, but suffer harder in terms of juggling due to having even worse landing options, or just being easier to juggle in general. Roy in particular certainly does not do very well against Rosalina due to his inherent design. Ike does better against her than Roy does, if only because her weight can allow for some early KO's given the correct reads, but if Roy truly did do well against Rosalina, why did Abadango use his
pocket Rosalina agsinst Ryo's Roy when his Mewtwo wasn't working, eh?
The Pits do decently well against Rosalina, thanks to their jumps enabling them to escape juggle situations better, but that's all there is to it. They have nothing exciting to shout about in the MU; their neutral doesn't particularly out-do Rosalina's, and their ability to safely deal with Luma is lacking compared to other characters without being punished for attempting. Pit and Dark Pit don't exactly win the MU, but their tools prevent them from outright losing, which is notable for them.
Yes, Link and Toon Link also have swords, but they do also have projectiles, of which are shut down 9 times out of 10 thanks to Rosalina posessing the single best move in the game at neutralising projectiles - that and her having a meat shield to soak up a projectile whose intent is to create an opening, only to realise using said projectile left an opening for Rosalina when Luma tanks the projectile. Link and Toon Link's swords are not used for spacing like Cloud, Corrin or even Marth and Lucina's are; simply possessing them does not make the MU's any easier.
Let me list off the
legitimate bad MU's Rosalina has for you all:
The MU's that Rosalina can struggle with (ie: even or a minor advantage to one character) would include:
All other MU's in the game are do-able; there are no other MU's in the game that should theoretically give Rosalina trouble.
I'm not trying to get everyone to change their votes on Rosalina, but Kirihara's win has been a perfect point to crush some of the speculation on her. It's shown us that she is not falling off in relevancy (and if anything, her good MU's with Diddy Kong and Sonic, two characters that not many characters can outright say they can compete with, are enough to prove her relevancy). As always, vote on what you all agree on, but I do hope I have at least taught some of you something you didn't know about the character. If anybody wants to ask any real questions about her as a character, do not hesitate to private message me; soon to be 3 years of using the character give me a broad depth of knowledge on the character.