The crux of Rosa/Ness as I understand it – with a focus on the Ness angle obviously - Ness has some excellent tools for this matchup and it's closer to even then it would appear at first glance.
When Luma is around, Ness in neutral wants to stay in a rough sort of range on the ground where he can space dash attack safely and use PK Fire without being hit by a burst option from Rosa. It sounds stupid that Ness even has to use PK Fire in the neutral as much as he does and I guess it kind of is, but it works because Rosalina has to respect it a lot. It’s a good way to put damage on to Luma and sets up a wall which Ness can use to limit Rosalina’s options. If Luma (or Rosa) is caught in PK Fire, there is very little Rosalina can actually do and Ness can go to town on the Luma with relative impunity.
Another important dynamic is being able to space Ness well in the air. If Ness is going to jump, he needs to jump so that he can react to Jabs with a falling up air/nair or drift backwards to safety and block a dash attack. This usually means Ness has to rely on fullhops.
Ness is able to get some mileage off of grabs even when Luma is around, but it is mostly a mixup and not the go-to since Ness can’t really do a Mario or Ike and use a throw hitbox to hit Luma away (dthrow has a hitbox but it does f.a.). What he can do is throw Rosalina up and then use the time while she slowly lands to address the Luma. Forward throw is used by some Ness, but in my opinion it’s worse unless Rosalina is sent far offstage, as it gives you less time to deal with Luma. The argument that it is optimal is that it puts Rosalina exactly where you need her to be (the corner/offstage). Which is great, but in my view, removing Luma is much more important at this stage.
What you don’t want to use is dthrow. Dthrow serves no purpose when Luma is around, throw combos do not work and Rosalina will reset to neutral too quickly.
Other tools Ness has to despatch Luma include Up Smash, pretty much every aerial, Down Smash, maybe Fsmash if you have time but you usually don’t, and sometimes jabs and ftilt/up tilt or PK Thunder if you’re on the ledge.
This gives us:
-Dash Attack
-PK Fire (which is so much better here in every way compared to other matchups)
-Back Air
-Up Air
-N-air
-Down Smash
-Up Smash
As the main anti-Luma tools. And most of them are really good at that job without compromising Ness unreasonably.
Obviously Rosalina can work around these options too but the main point is that Ness has got what he needs to play a sound neutral game vs Rosalina with Luma.
Once Ness has removed Luma, Ness’ real gameplan will kick into gear. Neutral is so much easier to play without Luma obviously, Rosalina is immediately limited in how she can contest Ness’ neutral tools. Ness now sets the pace and can open up Rosalina, bait or force reactions and get her into the corner or offstage using his standard neutral tools and his movement (mainly his initial dash and short hop stuff)
What a lot of Ness’ do wrong in the neutral is knock Rosalina away, then immediately go into shield when in fact, there is a very large window to hit Luma. Another common mistake is maintaining the wrong range and relying too much on shorthop aerials to remove Luma. They’re OK if they will send Luma miles away since all Rosa is going to get on Ness is a dash attack or dash grab, but people pick their moments badly. You don’t want to risk a dash grab if it’s going to send you offstage and force you to jump to get back, for example.
Offstage, Rosalina is basically dead. The aim is to use a move like PK Thunder to prevent an up B and force Rosalina into a Gravitational Pull (or Airdodge, or straight-up just falling) after she has used her jump, and then punish up B with the back hit of down smash (the window for which being very lenient because Rosalina pops above the ledge and lacks a hitbox on up B) to end the stock.
Simple yet effective.
If she does get back, transition to a ledgetrap - Ness is already on the stage, after all, and has some amazing ledgetrapping options which Rosa can do little to circumvent.
Disadvantage against Rosalina is pretty horrible and it isn’t just GP and up airs. Up airs are huge though. Because Rosa can float so low below Ness and use disjointed up airs it’s extremely difficult for Ness to get under Rosalina and get to the floor – ESPECIALLY if Luma is around to cover one side. Quite often, getting around it involves using an airdodge and drifting with the airdodge around Rosalina to try and get behind her, but this is only really effective if you read the up air. The key is only airdodging when Rosalina jumps, and not before.
Ledgetraps are strong against Ness, but Ness has one of the best anti-Luma tools for this situation in the game, in the form of PK Thunder. It’s not foolproof but it must be respected. If you can PKT Luma and send it offstage, dealing with the SoRo ledgetrap is… still hard but markedly easier.
GP gimps are stupid, but the amount of times I’ve seen Rosalina screw this up and die whilst Ness gets back is astounding. Also you need to be careful not to Pull PKT through Ness, because it will hit him and he can up B again. Ness does at least have more options to mix up his recovery vs Rosa than vice versa, where Ness can force a very specific recovery from Rosalina.
So hopefully that has cleared up Ness’ good tools for the matchup and might shed some light on why Dabuz has not listed it as a 7-3. Rosa with Luma screws Ness up and forces him to waste time in a matchup where one neutral loss can be the death of him. Yet he can play around the Luma super well and if he gets rid of it, Ness is going to set the tempo of neutral. Ness also only has to win neutral once or twice to end a stock… at least in theory. The matchup is always going to be in favour of Rosalina (and I would argue slightly more than 55:45) because greater ease of access than Ness to her (probably slightly stronger) advantage state, due to how Luma skews things. But the tides can change so rapidly in the matchup that it’s closer to even than it first appears.
If you want a good example of how Ness can play neutral against Rosalina with Luma, S1 is probably the Ness with the best neutral vs the character.
https://youtu.be/TJclpYCsIfM?t=2m39s (starts at game 2; Tatuman went Falcon game 1) I feel S1 could sometimes be getting more out of advantage state but the way he plays the neutral (with or without Luma), and his decision-making in neutral, is second to none as far as I’m concerned and I love that he focuses on getting Luma out of there.