If this thread wasn't so awesome, I'd call you a traitor for maining Marth and defending his so called "manliness," which doesn't exist. D:
Also, on the Defensive, remember that Snake can't instinctively duck under Aura Sphere, and even though grenades go through them, you can spam Aura Sphere faster than he can spam grenades, unless he tries throwing them as soon as he plucks them, at which point you just throw them back. Snake really is a tough project to stay defensive on, though, as most Lucario mains are used to their opponent naturally coming to them if they play defensive...
I'd never recommend playing purely defensive against a Snake, at any point in the match unless you somehow force him to lose his bearing and he becomes aggressive (in which case you just won the match, anyways, although if this happens, you're either a master at Lucario, or your opponent sucks).
I've fought a few pretty good Snake users, and I just have to say that a full, well-thought-out Offensive or Balanced style usually is the best way to play the match. If you give him time and space, he'll begin setting up the stage in his favor, and you'll have a tough time closing in. Just remember that there's a difference between a stupid offense and a well-thought-out offense, so don't just run into an Ftilt. That's stupid. Also, your Fsmash outranges his Ftilt(!), and if you're fighting a gutsy snake, don't whiff a usmash or roll behind him at any time(as usual, but this is even more important), as his fsmash, if given time to perform, is the fourth strongest move in the entire game (only D3 Fsmash, Ike's Eruption, and Ike's Fsmash are stronger, but these are only SLIGHT differences, like 2-5% fully charged). Luckily, it's the most rarely used move in his entire arsenal, although keep it in mind that some Snake's WILL go for it and you WILL die if you get hit.
All in all, keep up the pressure, but do NOT let him hit you, as tough as that sounds, as he has many very strong, very quick moves that will kill you (which means no aura boost!) if you let them.
BTW, I lost the matches against Snake, but they were pretty close, so I remembered what I did when I was winning. Also, they were much better players than me, anyways, so the mere fact that I almost won was a very amazing thing.
Luckily, Lucario seems to do much better against Snake than most people, so don't think the match is hopeless just because of that stupid, false matchup chart which thinks we have a disadvantage towards ROB and MK.
Lastly, Fsmash! Fsmash! Fsmash! Fsmash! Fsmash helps you so much in this fight it isn't funny regardless of your style! Fsmash outranges him, will do mondo damage, and means you're out of Utilt and Ftilt range!