Both dark pit and pit would like the room to avoid a lot of what ness may try, to large stages are good for us.
Ness has poor options to cover landing if you space properly for fairs and have the reaction timing (a whole half second or more) to shield pk pire, so a ness recovering high, or trying to aerially approach you will lose out most of the time. And following that logic, Ness can't approach us well. However, smart ness players will not commit entirely in their approaches. Rather, they will fade back as they use an aerial, or bait us, as @
Yong Dekonk
says. And since neither pit nor Ness have the best approach options, both sides will play a bait-heavy game with a lot of mind games.
That said, the neutral game will depend heavily on the players themselves rather than on the matchup. A few things to note, grounded pk fires are easily punishable by run up shield and then Ness suffers from end lag and we can do whatever we want in that time frame. Never use f smash in the neutral or anything remotely punishable like that. Ness gets a free grab and dthrow fair follow ups. On a similar note, try not to get grabbed at low percents since you will take a hefty chunk of damage quickly.
Ness can be juggled by us with our up airs, or up smash through plat forms, while ness doesn't have as easy a time with his up airs, but watch out at high percents since his up air can kill. But we can contest his up air with our dair, but the hit box on his up air is pretty big so proper spacing is a must. If you can't get it down, you are better off trying to come down another way.
Our roll is faster and less punishable so don't be afraid to roll away just to create space. We can use our rolls to cross up shield every now and then to punish some ness attacks, but don't do this too often as we come into grab range or f smash range and if you get read, you get screwed.
On Ness's recovery, his pk thunder can be reflected with guardian orbitars but I am not quite familiar with the spacing that goes with this, so this tech may be pretty hard. I don't know. I don't get to practice this stuff when I want. And I usually kill with a parry f smash anyways since ness is so light.
Even though we have a good f throw, we should not always try to kill with it. We have a killer f throw, and ness has a deadlier b throw. The set up we are looking for (their back is to the ledge) favors the both of us. If we whiff a grab, then Ness grabs us and kills us. Not to mention that ness has a larger grab range than us. Pester ledge with our presence. Do not commit. Wait for them to screw up. Sometimes run up, but then hop back or roll back. Fire arrows sparingly. Do things to force a reaction and then punish. We can punish consistently, but ness can punish harder. DONT COMMIT.
When ness edge guards us, he can nair at ledge to cover up b, so be ready to tech the stage. In fact, any aerial of Ness's can be used to stop our up B, but we must hope their timing is off. It depends more on whether or not they know what to do when the time comes. If we try to side b into ledge, if Ness charges a low angled f smash, we get hit. I think we can go low enough that his low angled f smash can't hit us, but this needs testing. Either way, we have a lot of jumps so we can bait an edgeguard (like charged smashes) and retaliate or at least get back safely.