It's an even MU and for good reason. But... it's hard to explain.
In any case, my advice is to play safe and space well. Lucario does a number on us once he gets close and I think he has stuff out of GR. If he does get in close, he often tries things like Nair ---> very little landing lag ---> all three jabs. If he makes the mistake of using his third jab, use OoS Nair to get him off your tail iirc.
Be SERIOUSLY wary of using normal Ness tricks on him. Using PKF to grab is a nono if you're at max range when PKF hits because he can almost always use his downB (a counter - I can't remember it's name) by that time and you can't grab him in his starting frames. Close up it's better but you have to experience that a bit for yourself and work out where you can safely go for the grab combo or try something else.
Also, another thing to be really wary of is Lucario at high percents. This is especially where you want to play safe. If you can get him offstage, it will be much easier because Lucario has issues with recovery (his upB does no damage as we all know), so you can take advantage of him there to use Bair or Uair or some other killing aerial. Remember that bair can be seriously DI'ed though, so be careful about staling it.
Watch for his AS. He often does it in a few situations, like when he's just thrown you away, when you're trying to recover from offstage, as a random mix-up up close or as a combo-breaker sometimes if he has the time before you hit him again. The most predictable of these situations are the first two examples. I've picked up some ridiculous percentage heal before because I read his edge-guarding AS when he was at about 140% (it was so beautiful ;_;
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Basically, Lucario has an advantage on us onstage at close-range. We have a slight advantage at mid and long range is pretty neutral. In the air I can't remember who wins. I think Fair is good against his Fair (outranges) but I can't be sure. Offstage, we can really hurt him, though I think he can also hurt us a lot offstage as well from memory.
Just, be careful of his tricks, learn his combos and respond with your own!
, the sandwiching! But yeah it's a surprisingly good team but you never see a Ness+Marth team anywhere for some reason.
