's Bair should go up. His Bair has the AC frames Fair wishes it had, and has range that rivals his Fair. It comes out very quick, at frame 7, is very safe on shield when tipped, or just safe wherever when used retreating. It KO's very early for it's general speed, range and safety, KO'ing at around the 110% (stronger than character's like Mario, Bayonetta etc), and the ability to turn Marth's direction around in the air is just yet another positive. You head off stage, but see Fox is going to recover straight up by the ledge. You want to use Fair because it's easier to time on an recovery move like Fire Fox. What do you do? Bair to face the right direction, so you can Fair him.
With those points in mind, I also believe
should move up too. However, she should be quite a bit lower than Marth. She has all the same positives as Marth, in general, but she cannot get the same KO power as him. While she doesn't have to space it near the tip due to the Parallel Falchion's properties, Marth's is still safe tipped or untipped, because of it's AC frames. Of course Marth's does have minimally more range too, which does become noticeable in a couple of matchups.
Now,
also has a similar Back Air, like Marth and Lucina. However, while I believe Marth should be somewhere near the top of B, and Lucina somewhere near the middle/lower end of B, I think Roy should stay at the bottom of C (he could even potentially move down to D tier, with a few others, but that's for another day). Roy's Back Air comes out 1 frame slower than Marth and Lucina, and because of how quickly he swings his sword, it's actually out for less frames than Marth and Lucina, who swing their sword from A to B slightly slower (Marth/Lucina: 7-11, Roy 8-10). On top of this, Roy's Back Air does not AC in a short hop, and due to the nature of the Sword of Seals sweetspot, even if he uses it, it is rather unsafe and hard to strike properly with because of the need to be virtually hugging your opponent to hit hard. On top of THIS, it KO's later than Marth's too. The main reason Roy's Bair is so bad, is because it is just unsafe. Even if it KO'd earlier than Marth's, it would still be worse, because of how unsafe it is.
TL;DR
C -> B (somewhere within the top half of, eg: I think it's better than Puff's and Zard's, clearly).
C -> B (somewhere in the bottom half, but it's still a great Bair).
C is fine (there's even potential for it to be down in D, but if characters like Pac Man, Dedede, Wii Fit aren't down there yet, I think Roy is justified at the bottom of C).
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Edit
Also, I want to touch on
's Bair. Rosalina's Bair alone is average; It's got average speed, decent damage, decent KO power (nothing to shout about, but it can KO), solid range, with some disjoint on the end. It does have a bit of endlag on it, and poor AC windows (still AC's in a short hop, but you get no variance).
Rosalina's real benefit from Bair comes into play when both her's and Luma's can be used together. When used together, it is essentially a multi hitting Bair, which is something special, unique to a very small bunch of characters. Together, their Bair is a relatively strong option, for multiple reasons. It has better KO power, thanks to Luma, and it's off stage potential is very threatening; while Rosalina's Bair alone is quite a scary option off stage, due to it's low angle of 38°, Luma's hits even lower, at 30° and of course, deals stronger knockback. It's essentially a semi spike, and can shut down a lot of characters or recoveries at very low percentages (Falcon, Ganon, Cloud etc). Of note, if you RAR their Bair, Luma's Bair reaches slightly further, due to him not being able to reach the front side of Rosalina before they leave the ground. And of course, you can utilise the Lunar Landing advanced technique to land using Bair. What's more, at low to mid percents, a Lunar Landing Bair can lead into a Dash Attack, or even a grab (where you can use a technique similar to Lunar Landing, where if you input Dash Attack, and grab straight after, Rosalina will dash grab, and Luma will Dash Attack, which is very good).
So yes, Rosalina or Luma's Bairs alone are meh, but combined, they become a pretty strong move overall, with multiple uses.