If I had trouble against Sheik, Zero suit Samus, Fox, Pikachu, and Yoshi as Rosalina, then the characters who I go to will be my mains or second secondaries. This is my personal experience with the MUs, but for those with my mains are
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and secondaries are
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:Mario, Pikachu, and Kirby are my go to for this MU because if I struggle with Luigi and Rosalina, I would use those three. Camping needles won't work against those three characters that much, and Mario and Kirby can kill better. Sheik can get combo by them too because of her falling speed.
: Pikachu and Kirby are her worst match ups because of their small size and crouch. Mario, even MU because of his combos, Cape, and better koing. I'm not sure why people think ZSS is a big problem for Rosalina, but I guess its because of her speed, Juggles, combos, and edgeguarding.
:Mario, Pikachu, and Kirby will be my to go along with Captain Falcon, but Luigi is a very difficult MU for Fox because of his combos, damage output, superior ko options, and edgeguarding. However, Fox still have camping lasers and speed going for him. Three of them can combo him and edgeguard, but Pikachu's and Kirby's edgeguarding are better though.
:Luigi will be his worst match up. Mario is part of it, but the MU is even and Luigi can more difficult though. As for Mario. Even though Pikachu's recovery is better, Mario will not let you get back on stage for free and deliver pressure with his Cape, Fludd, and Up-smash if you don't mix up your recovery options.
: I will use Luigi and Captain Falcon against the dino, but I also heard Zero suit samus can be a problem for him as well. I don't have a lot of experience with the MU Captain Falcon and Luigi because I mostly use Pikachu and Kirby for the MU and Yoshi beats them both.
As for Ike, he may have even MU or so against Yoshi, but against the rest on top, he can struggle.