you're better off just learning the matchup rather than picking up a third character just for marth. but if you must know, good marth counters are fox, falco, sheik and puff. falcon, peach and ganon all can play the matchup well with less skill if they know what they're doing. ics can also play it well but they're not the kind of character you pick up as a secondary given how different they are from the rest of the cast.
if you play fox and falco you'll have to develop really good di habits and learn to recover well, a very hard thing to do with spacies. basically i wouldn't recommend it since you're playing two mid tiers that will play very differently from spacies. sheik and puff on the other hand are perfect. of course you'll have to learn the matchups, but they do not get pooped on nearly as badly when marth gets a hit or sends them off stage.
if you're uncomfortable with sheik or puff for some reason, falcon, peach or ganon are your next best bets. falcon is similar to the space animals in the sense that you will get comboed hard and you're pretty much dead off stage. but you get to poop on marth whenever you get a grab and it's a very much playable matchup if you learn it. people used to think that peach was bad vs marth but things have changed. ganon is something many marths don't really consider but it's technically an even matchup. and in the hands of a player that knows that matchup vs a marth that doesn't know it ganon will have the advantage.
