Two major points here.
First, Robin's projectile game negates the speed advantage that these characters might have enjoyed against other swordsman-type characters like Marth and Ike. If all that they're doing is "running circles around" Robin, then Robin can just use side-b or charge up a neutral-b at no risk to herself. Her opponents have to actually apply pressure using hitboxes to threaten Robin since she has projectiles: they can't just run around (they can projectile-camp, but I'll get to that later). To approach, Greninja has to dash, Diddy has to grab, Sonic has to spindash, etc. And once they go for these options, they have to deal with Robin's own jab and disjointed tilts. Again, Robin's attack speed isn't slow, it's just her ground movement speed that is.
She doesn't have to approach most other characters if she doesn't want to. It's a catch-22 for them: stay away from Robin but let her charge Thoron and spam Elfires on your shield, or get in and face her disjointed attacks (which do a lot of damage for someone who kills so early, by the way--the risk-reward really isn't in their favor at all). It's kind of like facing DK or Bowser but if DK and Bowser were slow and had projectiles instead of speed, and were also not big targets. Can you imagine a half-sized but slightly slower Donkey Kong whirling barrels and coconuts toward you? It's like Brawl- DK! Just the thought is terrifying.
Second, minus Sonic, the characters on this list share one important trait: they don't improve much with customs. Their game with customs is about the same as it is without, but that's not true for Robin. Nope, I'm not talking about Thoron+. I'm talking about Fire Wall and Speed Thunder. While Thoron+ is indeed a great option in basically all matchups, Fire Wall and Speed Thunder improve these particular matchups by much more. Speed Thunder is good as an anticamp option, and Fire Wall is both a landing option and a quick anti-CQC move that conveniently also negates projectiles.
Now, with all this in mind, to dig into the matchups a bit:
I'd say that this is in Robin's favor. Sheik's needles don't do that much damage, so just take Speed Thunder in case she spams needles very quickly, and you're set against the camping. When it comes to approaching, Robin's huge disjointed Levin f-air beats Sheik's aerial options all the time, even if they are non-commit (Robin is like, "I'll MAKE you commit" and tips the scales over). Heck, even Robin's n-air beats Sheik's options due to the disjoint. On the ground, your dash attack beats Sheik's, and your jab and tilts beat hers cleanly too. Sheik's d-throw/f-air/bouncing fish/f-tilt combos seem scary until you realize that they're doing laughable damage, kind of like her needles, and that she needs to get you up to a fairly decent percent before killing you (130 to 160). Meanwhile, Elwind jab can do up to 14-16% (lol), and Robin is killing closer to the 100 ballpark. Sheik is not scary at all, imo.
I'd call this a 50-50 matchup. Greninja is significantly heavier than Sheik (almost as heavy as Robin!), and fast shurikens are quite annoying, even more so than needles despite inferior range. Thankfully, Electric type > Water type: Speed Thunder is great here (regular Thunder/Thunder+ loses to Shurikens). Greninja's low profile means that you won't be short hop aerialing against him, but why are you short hopping as Robin anyway? Shame on you, you heinous monster. Yeah, Robin's short hop is bad.
Full hop aerials still beat out Greninja's aerial approaches though, and on the ground your tilts and jabs have more disjoints than his tilts and jabs. Generally Robin is doing more damage than Greninja (3-4%) and also kills just a bit earlier with up-air or d-smash (both variants of jab kill too). They can both gimp each other (n-air or Elwind from Robin; Hydro Pump from Greninja...it's interesting that their best gimps on each other are their recovery moves, heh; also, Gliding Elwind is probably a bit better here, for what it's worth). In Greninja's court is that sick up-smash: it kills early, is disjointed as all get out, and DACUS with it is going to be absolutely monstrous (though Robin's DACUS will be a good tool as well aganist Greninja). Greninja's roll is very good from what I can tell, so crossing up is a bit of a problem in this matchup; just use d-smash smartly, pivot f-tilt, and Fire Wall! (Fire Walling behind you isn't always legitimate, but it looks cool).
Huh. You know, I actually think that Greninja as a character is better than Sheik. Go figure. But that's an argument for another time.
Now, I play a lot of Diddy, and I know that people think that Diddy wins this hands down, but I actually think this is one of his more even match-ups. Diddy loves to shield, and to shield grab, and Robin's shield pressure loves characters who try to shield a lot. Diddy's bananas get eaten up by projectiles, as does his Monkey Flip (at least for grounded Monkey Flip...for aerial Monkey Flip, use an aerial if you can read it coming, otherwise use retreating pivot f-tilt just in case he uses it: you give up a bit of stage control but it's almost always worth not taking 14% damage). You can't go wrong with retreating Levin f-air.
Fire Wall vs. Elfire is a close call here: Elfire + Nosferatu will give Diddy's shield a lot of trouble, but Fire Wall will eat through his Monkey Flip and banana game like no tomorrow. Speed Thunder vs. Thunder+ is also a close call. I would take Speed Thunder to further pressure Diddy's shield, but unlike the two characters above Diddy has tons and tons of kill options that you have to watch out for; your additional KO power won't really matter that much here, nor will your damage racking, so Thunder+ to seal the deal in terms of KO'ing him first is not such a bad idea. You can Thoron+ his side-b recovery (it's very easy to see coming, and if he recovers high you can up-air him), and if he tries to up-b you can catch him with a spike. He has a harder time gimping your recovery, though if you pick a stage with a wall that he can jump up, he can use that massive f-air to make you a very sad tactician indeed.
The most important thing to remember about this match-up is to not get grabbed. Do not get grabbed under any circumstances. Retreating options are your best bet, and I would shy away from Battlefield overall because it's so easy to get grabbed on that stage, and so hard to retreat. Any Omega stage or FD will serve you well; Yoshi's Island isn't bad either, though it does have that unfortunate wall. To get out of d-throw > up-air if you do get grabbed, though, just VI into Diddy, and then use b-reverse Thoron charges or Elfire/Fire Wall to land safely.
This is kind of like a mirror battle. You both have bad grabs, though Robin's is worse; you both focus on shield pressure, though Robin's is better. I think this is in Robin's favor. ZSS isn't very good up close against you, and doesn't have great damage options unless she gets you in the air: Elfire+Thoron > paralyzer, especially since you're doing more damage than she is off of confirms. Once she gets you in the air, her up-air is pretty good, but the main thing here is that you kill earlier than she does. You can't really gimp her, unfortunately, and her mobility makes it difficult to land an up-air on her, so I could see this being an even match-up (her n-air is disjointed and SH n-air is strong against Robin...though I've never tried out Fire Wall against it; does that beat it? if so, that'd be pretty important). ZSS' mobility doesn't really matter that much offensively, though. If she rushes in, you can jump or spotdodge because spotdodging the grab is very rewarding (though the grab eats through spotdodges in this game; you have to time it, or else you can roll behind her, since doesn't have a d-smash that catches rolls behind), and her other major option is dash anyway, which also loses to spotdodge/jump/roll behind.
Fox seems really hard because of lasers and reflector, and to an extent he is, but I want to point out that Fox isn't a tough matchup because he rushes Robin down; he's a tough matchup because he forces Robin to approach thanks to laser+reflector, and then he's very good at running away once she does approach. He'll build up damage on you really fast (avoid FD like the plague, or else this will happen even faster), and try to run away and shoot lasers as often as you can because you have problems getting in on him with your low ground mobility. However, Fox does lack in a reliable way to kill you or even to set you up for a kill via his throws: be wary of his up-smash, and although you'll be at like 150% or something, Fox will die to your attacks at 90% because he's super light (even lighter than Sheik!). As in every game, he's a fast-faller, so d-throw to up-air will be a very nice way to kill him (he can't really shine-stall as much as he could in previous iterations, either). Your attacks out-disjoint his in the air and the ground and you win up close, assuming that you do get in. 50:50, though, is what I'm going to call it, because while lasers and reflectors are really strong, the damage-racking is about on par with your early KO options.
Sonic does improve with customs, because Hammer Spin Dash is absurd, and unfortunately for Robin, this match-up is pretty bad as a result. Shield is a strong tool vs. Sonic in general, but Robin as a character isn't so strong when she's hiding in her shield. Both Thoron+ and Speed Thunder are good here, but Hammer Spin Dash will jump right over both of them. Sonic's pivot f-tilt and f-smash are so, so, so good. Meh, Sonic might just be the best character in the game after all, rather than Palutena or Robin. The more I think about his tools the less counteroptions I can think of. He's absurdly good.
You left out
, another really fast character, but I think that this match is in Robin's favor for many of the same reasons that it was in Marth's favor in Brawl. Robin can wall Pikachu out with retreating pivot options (f-smash, f-tilt, grab sometimes) or f-air. The only reason that I might call this even is because Pikachu can edgeguard Robin pretty well. It's pretty disgusting how well Pikachu edgeguards. Still, I don't think that Robin loses this at the very least: if Pikachu messes up his b-air edgeguard he gets spiked. Robin also kills Pikachu very early (Pikachu is as light as Fox).
You also left out
, whom I think is a big threat because he's got the best u-air in the game, even better than Robin's own because of how little it lags (6 frames of landing lag I think? absurd). It's so fast, and Robin doesn't have tons and tons of landing options. Also, he can grab you so fast out of a dash that projectile-camping him isn't always the best idea. Falcon is also kind of heavy, unlike most of the other characters on this list. Still, he falls pretty fast, so landing f-airs and u-airs on him is easier, he has a bunch of dead attacks like Falcon Punch, and he doesn't have an easy time edgeguarding her (whereas she can spike him; take Gliding Elwind imo, it spikes at a better angle for me, but your mileage may vary). I would take Fire Wall here because it covers all his options out of a dash up to you: shield, spotdodge, grab, and jab. I would also take Speed Thunder because he won't give you the breathing room to charge Thoron+. 50:50 at worst, though.
The reason that I think that Robin is among the best is because, in addition to not really struggling against these speedy characters, she has arguably very positive matchups against the likes of Rosalina, Jigglypuff, and Peach, who are top-tier threats. The matchups I'd be really concerned about as Robin are custom Ike and custom DK: when Robin is outranged AND outmaneuvered AND outpowered, we have a problem. DK's custom up-b and Ike's custom side-b, which goes through projectiles, are both very problematic for Robin to overcome because, unlike the generic ground mobility that you guys are talking about with regard to these other characters, Ike and DK (and Palutena, actually, and Sonic to an extent as discussed above) have quick dashing attacks that move them across the stage as they attack: Robin can't punish these very easily because she can't run fast enough to catch them at the end of the animation. Not being able to punish attacks that go through you is indeed a weakness of low ground mobility, but I don't think that "rushdown" is really a problem for Robin at all.
But I still think that because of DK's size and Ike's own slowness (punishing Robin's projectiles is difficult), those two matchups are probably close to even. Palutena and Sonic are probably her worst match-ups; everything else in my mind is either even or slightly tilted in one direction, or heavily in Robin's favor. Seems top 5 to me, but time will tell, as with everything else.
). It's not that Robin can't beat those nine in the MU, but that they just have an easier time going against the top characters.
