Ah, this MU... I'm a Ness main, and I can tell you I HATE this MU LOL. I honestly think this is one of Ness' worst MUs.
Vs. Kels
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The problems here (and yes I mean MANY) are that Ness cannot play how he's used to.
My playstyle is often going aggro and getting the grabs and starting combos and getting the follow ups. I ALWAYS throw out moves to cover myself. However, Ness' neutral is VERY lackluster. He can't fight at long range cause his projectiles have so much lag and are easy to see coming. He's slow and doesn't like approaching on the foe's terms. Also he has a gimpable recovery.
It's practically impossible for Ness to get a grab (unless Sonic whiffs a U-Smash but even then Ness has to be pretty close AND quick to get it) and he literally can't use his Specials like at ALL. You can't use PK Fire because Sonic will punish you for it AND even if it does land, Ness gets no follow up because Sonic can just Spring out of it. And he's not fast enough to catch Sonic's landing from Spring. PK Thunder is not an option either. x.x
Meanwhile, Sonic's top notch speed allows him to punish at almost any range. I also can't sit in my shield either because Sonic's Spin Dash does solid shield damage AND he will usually hit my shield safely (just watch how I tried to hit with an aerial OoS as Spin Dash was coming but BARELY missed as she jumped). Sonic's grabs are nothing to scoff at either and can cover Ness' landings pretty well with his speed.
Ness also can't recover too well vs Sonic. Spring eats up a low recovery (it will either hit him and knock him too far away to recover or cut the distance by half) and Sonic's Bair is pretty scary when you have to try and get back. He can use it to read a ledge jump as well, U-Smash also covers a neutral ledge get up.
Honestly I think one of things Ness has in this MU is Nair. A well timed & spaced Nair will allow Ness to beat out Spin Dash (it does require some decent timing), but I feel like that's Ness' main tool in this MU. Bair/Uair also work on occasion. Yo-Yos can also work decently in stopping Sonic's approach (but a whiffed one will get him punished).
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HOWEVER... Why the hell would Sonic have to approach Ness in the first place? He almost never has to LOL. Ness has an insanely hard time trying to catch him; therefore Ness can't get the grab for a KO or follow ups. Sonic can also rack up damage pretty well, while Ness can't do so too well this MU. Ness only gets a few hits in at a time, but struggles with KOing.
This (1:56 - 2:03) is probably a good of a string Ness will get. Like once in the whole match.
Regarding stages, I always like Delfino as a CP but I don't like T&C. I think Castle Siege is a decent choice for Ness as well since the first form doesn't give Sonic much room for Spin Dash shenanigans or to run around so it might be easier for Ness to "catch" him there and the ceiling is fairly high so Uair/U-Smash KOs are a bit later there (but Ness has to watch out for the walk-offs). Any other is a matter of personal preference, I know I'd ban Duck Hunt vs Sonic tho. XD
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Honestly it could just be personal bias since Sonic pretty much counters my playstyle (i.e. personal issues but also MU issues)... But I've always found Sonic so stressful. I do think I had some things going against Kels (best Sonic in my region
) so it's probably not THAT one-sided, but I'm willing to say this MU is somewhere around 65:35 Sonic's favor. I find Ness has to play REALLY on point and the margin of error is quite unforgiving for him to win since Sonic abuses Ness' lackluster neutral and recovery easily. x.x