In terms of matchups... Speaking entirely in <24 hours off the cuff...
I suspect he hates
, who is everything Sephiroth dislikes in a single character. Fast and low dash, threatening kill power that loves tall targets, solid mid-range projectile, and one of the best anti-Shadow Flare moves in the game that works at any %.
also takes Shadow Flare off the table, but otherwise hates being outranged and isn't dashing in on much. Probably good for Sephiroth.
doesn't wreck Shadow Flare as much as you'd think--it does nothing for them when they are at 0%, and Sephiroth is free to continue to apply pressure while they obsess over healing. More so for Ness, the odds might often end up in Sephiroth's favor.
Revenge is... pretty good. It's no Rebel Guard or Bucket, but it's probably the fourth best response after PSI Magnet. (And it works at 0%) You will only Revenge the first bolt, but you won't get hit by the other two. And Revenging one Shadow Flare is still enough for more than a 2.0x multiplier! Against a quick, single Shadow Flare, Revenge is probably the best. But note, Revenge on Flare is pretty bad, and Revenge on its bigger brothers will even still get hit (and wrecked) by the followup explosion. But Revenge is great agaisnt big slow swords, and Incineroar's low kill percents means less time dealing with wing mode.
Bounce is obviously amazing when you have it, and you have a little bit of time to fish for it when needed. While Hero really dislikes being outranged and outsped, his projectiles are pretty great for the range Sephiroth would prefer to fight. Probably a good matchup for Hero.
Pocket is okay. Obviously it takes the other Flares completely off the table, but that's true for almost any reflector. But while Pocket is a better-than-average defensive answer to Shadow Flare, it's not great. A single Shadow Flare is nothing to write home about even with damage doubled. And unlike other reflectors, Pocketing gives no coverage from the incoming punish Sephiroth was going to be trying regardless--not a relevant disadvantage for almost any other projectile in the game. Shadow Flare might be the one projectile where getting to use it later at a time of your choice
isn't better than reflecting it immediately.
That said, Slingshot is great against Sephiroth, it really shines. So I think the matchup still favors them? And having a "merely better than average" answer to Shadow Flare is still a big net positive, despite downplaying it.
Other low commitment reflectors are also a helpful tool against Shadow Flare but hardly take it off the table. For some higher commitment ones, it's less common that they will offer a meaningful improvement to your situation.
and
are also infamously low and quick, but don't have the brutal level of punishes that Arsene brings to any commitment. Both are also vulnerable to Shadow Flare, so it's less concerning.
I was worried that tall characters like
would have an especially bad time against SH fair/bair, but it's still too high; all tall characters are much shorter when dashing, which helps.
can out-do Sephiroth at his strengths, and even hedge his bets with Armor when Shadow Flare is on the table. Seems fine for Shulk.
has powerful mid-range stuff (like Joker or Hero) that Sephiroth probably hates. I have no idea if this will make up for the speed issue. One of the matchups I have the least idea of.
probably hates that sword, which seems like an amazing anti-float tool.
sounds favorable for Sonic? Going off my heuristic that Sephiroth hates run speed, I don't immediately see how it couldn't.
and
have the burst punishment to make the most of Sephiroth's commitments, like Joker. They'll hit hard and kill him early.
seems fine; he's super fast, runs low, and hits hard up-front. He doesn't care about Sephiroth's disadvantage tools, but he loves Sephiroth's low weight; "You mean I only have to land as many side-Bs as I do to kill Pikachu, except they are as easy to land as vs heavies?" Even Shadow Flare matters less in this matchup, because Mac exchanges are so fast and viseral. Less wing time is great too, and Sephiroth has fewer options than most for taking KO Punch off the table.
isn't so lucky. This just seems horrible for him. What are you going to do to get in, Wizkick? Ganon is just big, slow, and the perfect victim for Sephiroth to oppress.
and
probably doesn't have a fun time either, and I can't imagine
being happy.
hates a lot about Sephiroth. She hates uair, she hates u-smash, she hates fair and bair, she hates Shadow Flare. Pound against Lucina is one thing, but against Sephiroth-level disjoint it won't help you much. All her favorite edgeguarding moves get eaten by Octoslash, and he's not really jugglable. If she had Smash 4 SHAD, it would actually help a great deal in this matchup; alas.