At first I figured making something like this public would be weird but even if it is weird I'd rather be weird and learn and be not weird and not learn as much.
Basically I'm going to make this an area where I write down what I learned in a match up against my Ness and or what I SHOULD learn in a match up against my Ness and what I could possibly do to counter whatever it is. I thought of this just now because I sometimes carry a notebook to friendly sessions in real life if I'm struggling and it helps to an extent but the point of making this public is to hopefully receive feedback on what I learned to see what ever I'm realizing is true or fraudulent. I also just now played a really good Zelda main named Elf1985 and when ever we play we talk about what seemed scary about our matchup and I think that is a glorious thing to write down and reflect. This will be a somewhat advanced Guide so it is assumed the audience can understand the terminology. If not I recommend searching through ( https://www.ssbwiki.com/ ) for things that arent clear.
Zelda v Ness 
Me personally: 55-45 in zelda's favor
So My first official character to start studying is Zelda. I don't have much, I probably lost a lot of the knowledge flare I had writing that offal introduction. I find it will be best to summerize what I find out of the matchup just by analyzing the approach options and the defensive (Zoning) options because its hard to teach or understand neutral cause that all comes with how you apply your experience and teaching punish game or combos is too much work because that's something I'd rather lab or get out of watching tournaments. Plus I'm not too big on frame data or specific percentages.
(Note some of these advantages or disadvantages could be heavily player defendant rather than character dependent but it's still helpful if you point that out)
Step 1: Approach options
(Ness)
Ness normally has very limited approach options to begin with. His short hop is too floaty so it almost always has to be DJC'd with an aerial. Also his best aerial approach option in my opinion being his Fair it can easily be CC'd or Smash Di'd which is risky because Zelda has ton's of good CC options. Nair and Uair are normally follow ups or combo starters for when I'm ALREADY up close. Landing those attacks with an approach normally only works on a hard positional read which is difficult on a stationary character like Zelda. Finally Bair is hard to approach with because a full length bair to cover a significant amount of the stage would require you to do a late DJC or a DJC out of a gliding magnet which can easly be reacted to even by Zelda. What works with most match ups for Ness though is finding a way to CREATE an opening. Using tools like his movement (dash dancing, Breversing/Wavebouncing, or plat looping/wavelanding) or clipping with PK fire. Ultimately Ness for me works much better when the opponent approaches ME cause Ness has plenty of defensive tools but most Zelda's avoid approaching if they don't have to. On the ground my favorite 3 options on most characters are to approach with a well spaced dash attack, run up Dtilt/CCDtilt, or of course dash dance grabs. these are good mix up tools cause Dash attack is a great combo starter and has ridiculous range but it can be CC'd at high percents, but Dtilt and grab beats most CC's but requires you to be really close.
(Zelda)
Keep in mind that I don't play this character. I do have a lot of experience against it but it also doesn't mean I know anything detailed. I will spit it like I see it. Zelda is a very defensive character. I really only expect like 4 kinds of approach options. 1. Dash attack. It has similar properties to peaches dash attack, they are both REALLY good. Glorious combo starters, and I think it might also kill at death percent. However I do believe it can be CC'd and it also has a lot of end lag making easy to punish with OOS options like Nair. 2. Dash Dance grab, simply that. I feel like I shouldn't have to remind everyone that characters can do that but I feel weird not giving out a warning. 3. Din's Fire Zoning set ups. You will notice that if a Zelda CAN'T get in or is having trouble dealing with the opponents movement they WILL use their signature din's fire to narrow down your movement options. This provides a few ultimatums that normally just end up being A. accidentally run into Din's Fire and not only take percent but suffer a hard punish or B. work with the wiggle room and risk a KO move. If it seems you are able to deal with the pressured movement Zelda mains will often reactivate Din's Fire and make it return to her making the hitbox mobile again, pressuring you further into a more hazardous situation. That alone makes Ness's movement compromised because placing Din's Fire anywhere Ness can waveland cuts him off from good movement approaches. The best way of countering this is to either clank Din's fire, take the din's fire early (That's very risky), absorb with Magnet or read what the opponent wants out of Din's Fire (Much like understanding Falco Lasers.) 4. Teledashing which is basically cancelling the Up B animation at any time making it a zelda main's choice to go full length or any shorter length to mix up zoning options. I've noticed though that MANY zelda's will teledash behind you because it's much less dangerous than in front and then do a move based on your reaction. I've noticed though the best answer to this is just holding shield whenever you react to Zelda's teleport, wait for them to attempt to hit your shield with anything acting out of the teleport, then answer with Nair OOS cause Zelda has too much end lag on most of her moves out of teleport. However I haven't experienced it yet but I would assume that zelda's can grab out of teledash too so watch for that.
Step 2: Zoning
So far all I can say is that both characters LOVE Bair. Ness's Bair is shorter range but much more mobile and much faster, although Zelda's has more reach and is much more powerful. Not to mention that Zelda also has Fair which is kind of a Bair counter part cause it's used for similar things. Because of this Crossing up on Zelda is a bitch because she can end up bairing OR fairing you out of shield. I've learned to stay away from using pivot grabs in this match up. Ness's bair is broken against zelda however cause you can do rising DJC with Bairs and safely reach top platform of most platform heavy stages over and over again. The only down side is that Zelda COULD (I'm not sure) control center stage with a well timed shield stop and then Ness is screwed because he is above the enemy and doesn't control stage anymore.
Also Zelda Has her Neutral B witch is like a long lasting unjumpcancellable shine with multiple active hitboxes that make it impossible to approach with out perfect spacing. This can be punished hard though if its whiffed so pay attention to when a Zelda Main might use it because it probably means they are either in a pinch or very afraid of getting hit.
Ness's other zoning tool that I'm pretty sure I'm the only Ness that does it because it's gimmicky is the upsmash! I like using it against floaties as an anti aerial because characters like Zelda and Peach that try to approach from the air or try to fall with an active hitbox get knocked up again no problem for an extended combo!