I struggle with this MU, and I'm working on Olimar for it just so I don't have to deal with all the ways Fox can lose it so quickly. New England has a lot of good Ness players and I've gotten to know the MU, one of my main practice partners is a good Ness.
Basically if you're offstage, you might die. Ness always has the option to spike you out of side b or up b, but the Ness mains I play usually opt to to go for a down smash at the ledge because it renders recovering to the ledge with side b completely useless, and will kill at around 70 if they charge it. It SUCKS. So most of the time you have to up B to the ledge, but then you have to watch out for PK Thunder and a well-angled PK Fire as well. It's a huge disadvantage and the only advice I have is to save your jump, and use your shine stall to try and see what edge guard attempt they want to go for, then react accordingly. One tip for the down smash edgeguard, if you recover high you can just side b over the yoyo and hit Ness while he is charging, if you didn't get knocked too far off stage.
Next, there's the fact that Ness' aerials all have ridiculous hitboxes, in particular Nair and Uair. This renders a lot of followups useless, but many Ness players throw out the aerials predictably to cover their landings, so you can just punish it, usually with a grab because if they space right they can be in shield before you get close enough for a dash attack. The main problem is Nair just makes it really hard to combo Ness, but, for instance, if you're trying to get an up tilt string at low percent, you can still get multiple up tilts by reading where they will try to Nair out of it, power shielding, and then throwing out another up tilt. I find that Ness players are more likely to try to Nair (similar to Luigi or Yoshi) out of your combos then jump out, so even when you're chasing Ness down for the next hit of your string, a well placed power shield on that predictable Nair will give you some good options. The Uair has ridiculous range too (it can outspace up tilt) and hits hard so watch out for that, but you can punish it if you shield it. I don't find that landing damage on Ness is that hard.
As far as not taking too much damage yourself, the first thing I can think of is to DI upwards and behind Ness on his down throw at low percents. He can still true combo a Nair out of it, but it's better than taking four fairs and getting sent offstage, which is what will happen if you try to DI away. Fox falls so fast that PK Fire is not an issue, you can roll out of it, or even jab through it, basically getting hit by the fire cuts off the endlag of moves, so you can like uptilt or jab faster if you're getting hit by fire, it's good for Ness players who try to read that you will roll out of fire, but not good for Ness players who are gonna try to hit you with the tip of that OP baseball bat Ness has. Around mid percent, like after 40, Ness stops having many combos, so he mostly has to rely on his aerials for damage. Some Ness players like to dash attack, which is very punishable, but most will just try to space Bairs and Nairs and get the grab that sends you offstage. Learn your paticular Ness' air movement (this will come in handy when it's time to kill) and punish those attacks, and you should be able to get a percent lead on Ness mid-stock without too much trouble.
And here's the real struggle of fighting Ness. When it's time to kill, as in when you have Ness at over 100%, the Ness player will probably wise up and start hanging back, just waiting for a chance to get a rage backthrow. This means that they will be shielding a lot, looking for that shield grab, and we all know how much it sucks when our opponent is shielding a lot. I have very little advice here, because it all depends on the Ness to make a mistake. Be VERY careful with your aerial approaches, and always try to cross ness up so that you can't get grabbed as easily. Side B is an ok option as long as you land behind them in case they shield it, watch out for short hop back air or nair out of shield there too. If they still stay in the air at all, you can try to trade an aerial with an upsmash but Uair and bair outspace it so be careful. If the Ness is good, there's basically nothing we can do to get the kill, as far as I can tell. Most of the time I end up just getting a lot of throws until Ness is around 170 and an up tilt will kill, but there's very few attacks it's safe to throw out because of the chance of getting shield grabbed and thrown. You can try to grab and up throw ness, but most of them are gonna be smart and grab the ledge to get back onstage, making it for you to Up air them, but if you've caught onto their air movements or can read an airdodge, it's pretty safe to go for. That said, the one thing to take from this section is that the only way you're gonna kill Ness is if the Ness player makes a mistake. It's sad but it's true.
This final tip is a bit crazy, since everyone will tell you not to go offstage against Ness and for the most part they're right, but Ness is actually in a lot of danger offstage after he's used his jump, and his up B isn't great, so in moments of either great confidence or great desperation, you can go for the fast fall fair to drag him down a bit (it doesn't take much, his recovery gets bad height) or you can try to reflect his up B before he hits himself with it. Ness will usually wrap the Up B around himself going away from the stage to knock him towards it, so you want to jump over to the far side of Ness and then down B. For both options, SAVE YOUR SECOND JUMP and be ready to tech in case anything goes wrong.
I think that's all I've got. Again, I struggle with this MU and I'm gonna be working on it a lot, since I only just recently realized how much I struggle with it. The alternative, and I'm dead serious here, the alternative is to get a decent Rosalina. Like if you just play Rosalina for a week, she's so effective on Ness that your decent Rosalina should be able to handle a good Ness with ease. It's not what I do, but like I said, I've opted for Olimar, because I think he covers all of Fox's worst matchups really well.
Good luck!
). Fox however, has the speed to overwhelm Ness and is not easy to grab unless Ness gets a good read or Fox overcommits. Conditioning and stage control are essential for Fox here!
