Ah, finally, someone else like me and Aevin who wants to main both
Be careful with how you play them. You may forget that you are playing one with the other. They play nothing alike. Lucas relies on his pK fire and tilts and uses his smashes to kills. For Ness, this is not the case.
Ness' game revolves around his godly areials and dustructive grabs. His fair combos, has good range, and priority. His bair is a monstrous killing move, and his uair juggles and has HUGE killing potential, much more than it seems (It kills just as easily as DK and Wario's uairs). His dair is one awesome spike. It has a VERY decieving animation as the hitbox is rather large compared to Ness' tiny legs. And nair is a lagless, get out of my face move. It combos nicely and has long duration and comes out quick. It even has killing potential off stage making it a great edgeguard. All of Ness' aerials (minus the dair) combo easily into one another and can be used repeatedly. Simply flow from one aerial to the next utilizing his unique jump.
Since Ness plays the airgame, use his moves to send opponents skyward (a lot of his move do this). The best tool for sending enemies upward is his usmash. IMO, its one of the best in the game, but I'll go into the specifics later. Simply hit them with a usmash, charged or uncharged, and they go up. The utilt, dsmash, dair, uair, and dash attak also sends opponents up. If you grab, a dthrow always sets them up nicely, and so does the uthrow, but I prefer the down.
Onto Ness' yo-yo. Like I said, I think its one of the best usmashes in the game. It has range (it can hit people from under the battlefiled platforms), priority (it can hit ROB out of his dair), duration (all the time you charge it, its has a hitbox), and good knockback (not enough to kill, but sets them up in the air.) It has mindgames too. Charge it up, and watch your opponent walk into it. For some reason, people underestimate the charge time. If they use a projectile, the yo-yo can eat it while it charges, and it shield Ness from regular attacks too. If they DO use a regular attack, they stop in their tracks, and you release your yo-yo, popping them up into the air.
It can also be used on the edge. If the opponent grabs the edge, dash at the edge and usmash out of dash. This will dangle the yo-yo off the edge and no matter what the person does, they WILL get hit. If they attack, get up, or jump, the charge will hit them. If they roll behind you, release the charge and hit them with the backswing.
For throws now. Ness' throws are excellent. His dthrow combos nicely into whatever aerial the opponent DIs into. Sometimes, they can escape, but then that's where mindgames come in. People usually DI away from Ness and land on the ground, setting up for a usmash or a dash attack. If not, they will be in the air waiting for an fair, uair, or nair.
And his bthrow... wow. This is one powerful throw. Its the strongest bthrow in the game. Kills anyone anywhere on FD at about 130%. KILL
Lastly is the PKT mindgames. PKT is a mindgames heaven. The bolt harasses people while the PKT2 kills. It would take a million years for me to describe them all, so go watch some pro Ness vids to see what you can do. I reccommend .Yoshi.