Okay. I always lump them together as lots of other people do, but I can see your reasoning.
I'm gonna tell you that if you play Lucas like Ness or vice-versa you WON'T have a good time.
Ness main here (name's Timmy), I WILL admit it's been too long since I last played the MU, let alone offline (and my region lacks Kirby lol, I got maybe 1 or 2 Kirby co-mains here), but I do know a fair bit.
I DO however have a solid set (old and on 3DS lol) vs a really good friend of mine who mains Kirby tho.
Check it here! (5:25 - 9:10, 11:14 - 14:56, 16:16 - 19:37)
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So basically the idea is for Kirby to try and stay grounded, and bait Ness' approaches since Kirby has some deceptively quick tilts and landing lag. If you jump, Ness WILL wall you out with Fairs and/or trade with Bairs (and Bair is safe unless powershielded!). His Nair also outspeeds a good majority of Kirby's aerials (your Bair is 1 frame slower than it which is your quickest aerial). Any other aerial will get outsped and we tend to win the trade with Nair too lol. Kirby is also really light so we can't reliably use PKF (he always jumps out of it lol), and can bait it with his multiple jumps. I really don't think PKF should be used in this MU like at all lol. But since he's so light, he will die pretty early to B-Throw and one of our other beloved KO options (D-Smash/Bair/Uair/Nair etc.). Kirby is a pretty easy target to PKT juggle as well due to his lack of air mobility, but we have to make sure he's not gonna be at the point where he can just Stone through it (Ness can either hide under a platform and PKT or just PKT2 away if PKT is in range and Kirby's about to fly above him).
Kirby can gimp Ness with a Dair if he's not careful, and a nice F-Throw or B-Throw will send Ness offstage. And again, his Tilts should be used to bait out Ness' approaches (which Ness is pretty bad at). We technically aren't forced to approach either though, so you can expect this MU to kinda drag on. Although I'm a pretty aggro Ness myself. =P
Capitalize on Ness' lack of solid landing options, space well and bait him. But remember - Ness punishes VERY hard and can end your stock early.
I wish I could contribute more info tho lol. I'm willing to say 55:45 Ness' favor. I don't think stage choice is a huge factor, but I DO ban Lylat since Ness isn't great there (tilting can affect projectiles/recovery). It's personal preference (platforms vs no platforms) tbh.
Hopefully I helped, feel free to ask me any questions about Ness in general though
