My brother plays Ness all of the time. I have some experience versus Fox, but not enough to have answers for that MU.
Ness has a few things to watch out for:
Sparkles (his f-air) when you are at low health, he can easily combo with them after a dthrow or just spam you with them. Just avoid them the best you can, I air-dodge after the throws and it helps a lot.
His B-air as a kill move and a damage move. He can use 2 of these in a row before he hits the ground, and that can be just as devastating as the sparkles combo when you are at low percentages.
His N-air is amazing, if he just runs off the stage when you are trying to recover, he can bank you into the stage and spike you. You just have to be smart in how you recovery, but that is a really strong option versus Link.
His U-air kills very soon, I guess if you are in the air at like 100%, you should be weary of this aerial. I am pretty sure your D-air will own his face, but that move is like DK's u-air, so be aware of its beastliness.
His D-air can gimp you, but I think after you get used to playing Ness, he is going to get a lot less of these, just be aware that he can combo with it, and possibly kill you like Ganon's d-air will. I think this is his least threatening aerial, and possibly the most threatening aerial.
His F-smash sucks, if he does this just block and F-smash him. He might reflect an arrow, but w/e.
His Dsmash and Upsmash are both situational. I think they are more useful than his F-smash, but they are only really good when he can not hit you with anything else. They have some combo strings, but I wouldn't be worried about these.
His fire can lead into a dthrow-> sparkles combo, or a b-throw kill so be worried about that.
DO NOT hang on the ledge to long and let the pikafire own you. If it does hit you, you have lost your second jump, Up-B until you get to the ledge again, otherwise you are dead.
His aerial pika fire, is also kind of meh. I sure it has some uses, but my brother never really uses it, unless he is trying for the same combo as the ground pika fire leads into.
His absorb move and pika flash are pretty useless, who cares.
His Pika Thunder is very gimpable. Any of our projectiles can kill the ball, we can jump into the ball, we can jump into him if we are low and watch him die, we can just hit him during that recovery, we can gale him away from the ball, etc. Ness' recovery might punish us, but it is his most vulnerable moment, so I would definitely say punish the **** out of him.
His Throws.
His F-throw is a set distance, so he will never kill you with it, he will just throw you off the stage with it and go for a gimp.
His B-throw will kill us at 120ish... You can DI it some, might want to practice that, but it will kill us. It also gets a weird meteor spike throw every now and then, and he get a horizontal throw off, if this happens you might just lose a stock. One of those tough **** moments.
His D-throw leads to some sparkle combos, be ready to airdodge, and also after he sparkles, be weary of a bair. I generally air-dodge until I hit the ground, then block.
The U-throw, I imagine it can be used for u-air strings, Pika Thunder mind games, or used as a Vert kill move. I do not see this move used very often, or at all by my brother or my other 2 friends that play as Ness.
The headbutts are annoying, and I am not sure about his grab-releases versus Link.
His Dash attack is pretty decent, it has a fatty hitbox on it, so be careful about that, it lauches you up, so be ready for more sparkles after those sparkles hit you.
I think this MU is slightly in Ness' favor at around 55:45. He has a good close and aerial game, which tends to give Link some problems. He is also small, so I have some issues approaching with the ZAIR.
He does have a bad recovery compared to other small characters and has some issues getting to us.
55:45 Ness' Favor
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