Marth
Recommended Counterpick: Corneria / Brinstar
Let's give a friendly shout-out to Marth, your co-Fire Emblem character. With the introductions out of the way, let's just see what is scary about Marth, shall we? First of all, he appears to dye his hair, so he can never hope to achieve Ike's pure blue coloring. Actually, what IS scary about him is that not only does he have raw speed, power and range all perfectly placed into one character, but he is also one of the few characters who can gimp Aether, so you're going to have to hope that your edge game isn't the only good thing about your Ike. That said, Marth has his own issues, which will be covered next.
You're going to have to win this by exploiting Marth's weaknesses. Unfortunately, Marth is a pretty good character whose weaknesses aren't easily used to your advantage. First of all, similar to Meta Knight, Marth weighs as much as a feather does. Ike has the power to consistently kill Marth at low percents, the main issue is, however, landing the blows. Marth also has no safe kill moves. What does that essentially mean? It means that for Marth to kill you, YOU are going to have to mess up. It's a trying match, since Marth is also good at punishing mistakes. The solution is, of course, to play perfectly. If you play aggressively, Marth is going to utilize his 4 frame counter to counter your attack back for 1.1x damage. If, however, you stick to defense, Marth can then just pressure you with Shieldbreakers and Dancing Blades. The key is to, essentially, keep Marth guessing. Don't always instantly unleash your attacks whenever he's in range either. Much like you might use the airdodge to bait an up smash, you use some waiting time to bait the counter, and hit with any of your powerful moves. Be sure to note that Marth's recovery can be edgehogged with good timing. If you take advantage of that, you'll gain the upper hand.
Of course, our fellow Fire Emblem character is not to be taken lightly. The first thing you should be trying to do as soon as the match starts is to not get grabbed. The reason for this isn't a chaingrab, but the fact that Marth has a combo that goes: Fthrow->Tippered Fsmash. Marth's signature Forward Smash is a beastly move, and this combo'll deal near 25% on you. It won't work at later percentages, but it's always a good idea not to be grabbed in any match. Speaking of 25%, that's how much a NAir to AAA jab does, and you should be using your Neutral Aerial much more than your Forward Aerial in this match, though that doesn't mean you shouldn't mix up your options, of course. The general idea is to build up damage, then bait a wrong move and destroy our friend at obscenely low percents. And, on the topic of building damage, try not to leave any lag for Marth to exploit, since he can instantly punish you with a Dancing Blade combo that can do up to 18% I believe. He'll generally do the standard variant, but don't get too confident by overshielding and side-stepping the last move. Why is this? Because that last move can be the multiple hit down version. If you get hit by that, it's not terrible, just DI. Another thing to note is that Marth has a famous sweetspot tipper at the end of his sword. This means that his attacks have increased knockback and damage if he hits you with those. The logical thing to do, then, is to get close to Marth. Your jab, jab cancel (Though look out for the Dolphin Slash!) and grabs should all be used interchangeably so as to rack up damage and harass Marth.
Now that you are pestering him, he decides to take to the air, in hopes that your aerial game sucks and you'll die there due to his superb juggling skills. Unfortunately, your aerials most certainly do not suck and you've got ground based options as well. Remember that you outrange his Fair with yours, but be wary of being predictable. If he decides to retreat backwards, you can hit him with a dash attack. The main thing to remember is that Marth has a blind spot right below him in the air. This means that when you get him there, you've got plenty of things to use on him and all he can really do is Counter. If he tries to counter, just charge past the counter frames and hit him with an even more powerful attack! Now, Marth is recovering (Let's pretend that you knocked him sideways). His Shieldbreaker can help him recover, but it's predictable, so punish it. The Up B only goes up, so Eruption can teach him a little lesson in that. If he gets ledge happy, you've got a very fine Dtilt spike you can use against him. If, let's say, you're the one recovering, make sure not to get predictable. Save your second jump so you don't have to deal with Counter/Aether mindgames much. If Marth seems to be fairly close, he may try to edge hog the reverse Aether, just be wary and plan accordingly. Your best idea is to just try and get back on with great DI and a little sense, as over the edge Marth has more control than you do. On a last note, in this match-up, sometimes the basics work really well. Simply walking back a bit and attacking with say, a forward smash, is viable and will kill Marth quite fast.
Choosing a stage for this match-up is actually kind of hard to do. For all practical purposes, you're rather similar when it comes to stages. Just choose what stage you play best on and go from there. If you must choose a neutral, Halberd works well enough. The low ceiling helps you kill even earlier, and the neutral platform design punishes Marth's up b to the extreme. Counterpick wise, you've always got Corneria, your trusty side-kick. Brinstar also works, due to Aethersliding.
On the whole, Marth's got the tools to punish you, kill you early and gimp you. On the other side of the coin, Marth also has predictable kill moves, a predictable recovery and a light build. Using these to your advantage will allow you to rob Marth of his stocks quite fast. The main idea is to not be predictable and to stay away from the sides of the stage at all costs.