Hello. This is a matchup I'm pretty well versed in so I'll offer a little input.
I feel the matchup is pretty even, Rosalina has good tools to wall out Greninja when Luma is around but Greninja has good tools to eliminate Luma. Rosalina is approaching in this matchup because GP is not fast enough to eat all of Greninja's new, faster shurikens.. however Greninja can be punished by getting too spam happy with these, as they are only really good for some chip damage on Luma to ensure it is knocked away when hit with another move. Since Rosalina won't be getting hit by them, it's easy to run in and punish a shuriken-happy Greninja. At mid range be cautious of GP, because Greninja is fast enough to punish it most of the time. Sometimes it's better to just block the shuriken if he charges it.
Another thing of note is Greninja does not have a throw to get rid of Luma. If he grabs you while Luma is around you can punish his attempt to up throw -> up air you by calling Luma in mid air. A smart Greninja player will wait and block this though, and Rosalina being above Greninja is incredibly dangerous for her (as it is in many matchups). So, still expect to be thrown upward.
Greninja's tools to get rid of Luma consist of his jab, ftilt/dtilt, smash attacks (which is not always a bad option since Luma extends the hitbox and can snuff out an early punish attempt), and his best two are dash attack and short hop fair/nair. Dash attack has poor range but pops Luma up and away, often off the stage. If Rosalina is recovering dash attack -> fair is a true combo that will get rid of Luma almost every time. It also has low endlag, and Greninja can often get out jabs to beat punishes if spaced well. Greninja's best tools in this matchup are without a doubt ff nair and fair. FF nair when spaced well lands between Rosa and Luma and separates them. If it hits Rosa she is popped up for easy followups such as up smash or fair. If she blocks it, the attack has some shield pushback that prevents a heavy punish. She can potentially shield grab this, but without Luma being grabbed isn't nearly as threatening. Fast fall Fair is even better and safer, and knocks Luma for heavy knockback while being very hard to punish on block or dodge. Greninja will be using these two aerials in neutral a lot.
These two characters juggle each other like crazy. Greninja is a light fast faller, which means Rosalina can juggle him with relative ease with up air. Because Greninja's jump is so high however, most Greninja players will choose to jump instead of pick an air dodge. If you catch his jump with up air he is going to take a lot of damage or lose his stock, but Rosalina struggles to get up that high most of the time. However, Greninja lacks a quick response aerial outside of bair (which he has to be in position for) so he cannot really punish whiffed aerials when he is also in the air. On the polar end, Greninja also loves to juggle Rosalina at early %. Up air beats out a lot of moves with slower startup, and his up smash at worst will trade with Rosalina dair and can KO around 80. Do not challenge it.
Edge guarding is pretty even, but Greninja has the ability to mix up his recovery much more than Rosalina, and in this matchup will need to if he wants to stay alive. Rosalina can easily intercept him with dair or place Luma below the edge to catch low recoveries, and use up air/up smash to catch him if he misreads and goes high. On the opposite end, Hydro pump really messes with Rosalina's up special recoveries and can often cost her a stock. If Rosalina recovers high she's going to get hit with something heavy anyway, Greninja will not be chasing you offstage very often due to most of his aerials being poor for edge guards (you will have to watch for RAR bair though). Shadow sneak is a decent mixup offstage due to how hard it can be to see coming, but the risk is often not worth it. Just keep it in mind.
And with that novel written I think I'm starting to run out of things to say. Greninja's mobility and great juggle game help him dance around Rosalina and keep her airborne, but Rosalina has the ability to juggle Greninja right back and has a safer neutral. At worst I think this is 55:45 Rosa's favor but I will submit 50:50. It feels the stronger player will win out in this matchup more often than not.
