The truth is that Rosalina would already be a good character without Luma by her side. Her basic tools are somewhat straightforward and not that hard to use. Her recovery, her aerials, her dash attack and her throws are all at least very solid and serve a clear purpose. I'm pretty sure not a single of her moves is useless. I wouldn't be surprised if all of her moves were good in some way. And it all doesn't really have to do anything with Luma.
The front hitbox of her nair is still impossible to punish if Luma isn't there.
Just because her potential seems to be almost immeasurable doesn't mean that she's difficult to pick up.
That's a huge exaggeration. I don't know where a lot of the community gets this idea from, but Rosalina has very definite weaknesses without Luma, and most definitely wouldn't be a top or high tier character without Luma. Probably not mid-tier either. And I wouldn't exactly call a mid-tier character "good" if that's what you were referring to instead.
Yes all of her moves when Luma is gone "have a use." 99% of moves in this game have a use. However, no that does not necessarily make them good moves.
The hitbox on her uptilt is only present on the ring. This makes it incredibly weak both as an anti-air move and against grounded opponents because she can't cover just about anything diagonally. Anyone who gets hit by her uptilt would be hit by just about any other move anyways, so there's really not much reason to use it when Luma isn't there.
The hitbox on her nair is only on the feet. While it has low cooldown at the end, it takes a while for Rosalina to swing her feet around her body in order to get it into that position. So you can smack her when she makes the initial swing-around movement, before the hitbox reaches her front side, or even afterwards with a jump-canceled move out of shield. The way I see most people try to punish Rosalina's n-air is the same way people tried to punish ZSS' down smash upon Brawl's release, except Rosalina's n-air has way more start-up, no frame advantage, and overall far less use cases. Rosalina's n-air without Luma is somewhat strictly a zoning tool.
F-air takes too long to be very usable in a neutral position. Yes, this is a minor point in a vacuum. However, I bring this up for 2 reasons. Firstly, her air game from the front without Luma is incredibly risky to use when you're trying to make it back to a neutral position from high up. N-air takes a while for the hitbox to reach anywhere in front or below her. You may as well time a b-air if you're going to try to use n-air while being juggled. F-air has considerable after-lag if you don't land with it. And d-air only covers directly under her. Consider all of that along with her floatyness and you can reasonably understand she becomes much more easily juggled without Luma's hitboxes there. Secondly, If you consider her poor front-ward air game, and the small hitbox on her uptilt, she has a pretty blatant weakness against aerials and any characters that use them often.
Next I'm going to list some aspects of Rosalina's gameplay that are removed when Luma reaches 52 total dmg, is knocked away from her, or is knocked offstage. Keep in mind some of these weaknesses are realistically dwarfed when you consider she's got a projectile absorption move with Luma coming back on a timer. But we're theorizing Rosalina without Luma permanently, here.
- Much, much fewer of her moves combo into each other when Luma is dead.
- She loses her projectile.
- She loses literally half her range.
- She loses hitboxes in areas that normally cover the angles and timings Rosalina by herself is particularly weak against.
- She loses her zoning game.
- She loses any kind of decent offensive pressure.
- She loses combos from a grab.
- She loses a little bit of kill power on her smashes.
- She loses combined damage from moves that would normally hit with both Rosalina and Luma's hitboxes.
- She loses additional safety on block on every move that had it.
Edit: Her basic kit reminds me of Brawl's Zelda, kinda