I don't want to eat too much crow if that turned out to be true, so I won't deal in any absolutes. However, Robin is a Fire Emblem fanboy's pipe dream. If I had to quantify his or her chances, I'd put it at one percent.
I agree with this, mostly. 1% is harsh, but Robin is certainly a pipe dream.
So far, Sakurai has cared little for non-identity avatar characters or commanders in Smash. It's why Pikachu was playable long before a generic Pokemon Trainer; and even then, the PT is more of a background decoration. It's also why tanks and soldiers were the Advance War assist instead of one of that series' COs. It's why the Wii Fit trainers are representing the casual era and facet of Nintendo rather than Miis, as far as we so far know.
I'm not going to pretend like the moniker of "causal games" means anything to me, but if Mii games and Wii Fit are causal games Animal Crossing is as well. Animal Crossing's Avatar character got in. Using the "no Avatar" rule, after Sakurai himself broke it is pointless.
Sakurai focuses on the characters most seen and admired, and in the case of Fire Emblem Awakening, Chrom and Lucina outrank the rest of the cast, because they're the main characters and their story is more compelling than nameless, faceless Robin. I am not certain that any Awakening character, even Chrom or Lucina, will make it in.
Chrom and Lucina outrank Robin in terms of popularity, sure. However, Robin is seen just as much as Chrom and far more than Lucina in game. Outside of the game, they represent Awakening a bit more. Despite what you may think, if you play Awakening you will see that Robin does have an identity. Robin is given a distinct personality, including hobbies and interests. On a similar note, by playing Awakening you see that Robin is actually a main character, a very main character. Robin is of far greater importance to the story than Lucina and is on equal footing with Chrom for main character status. As for their story being more "compelling" than Robin's that's sort of opinion. But the only separate storyline for Chrom would be chapter 10 and below, which is honestly some of the worst FE writing I've seen. After that, the stories are intertwined and hard to separate. If you had to, Robin's story is essentially the game's entire plot.
I foresee a counterpoint to this will be that the Villager made it in despite being a generic player avatar; but it still fits the criteria of what character you see the most in Animal Crossing; a warm-smiled kiddo going about his mischief with shovels and slingshots. And there is also the fact that Olimar, a commander in his own games, is playable in Smash. Again, it's who is constantly on the screen in Pikmin, and he's interesting.
Robin gets the most screentime in the game. Or at least as much as Chrom. Spoken like someone who did not play the game, he's on screen
constantly. He's also important beyond the entire story of the game hinging upon him, Robin is the group's Tactician. (The one who commands the army.)
So I discount this rumor and any others that contain Robin, or Andy, or any human Pokemon character who fights with his own hands. It's just not in line with what has so far prevailed in Smash, and it smells of non-professional thinking.
I understand that a Trainer fighting seems stupid, Sakurai draws from the Pokemon games and the Anime (The Pokemon Adventures manga is where the trainers fight too, and, sadly, Sakurai does not draw from that). However, I fail to see how Robin or Andy are "unprofessional" ideas. If this is predicated on you thinking Robin doesn't fight like Andy, than I understand where you're coming from. But Robin does fight. As for Andy, he commands armies. Not that much of stretch to make him a fighter.
And what has prevailed in Smash is giving non-fighters fighting moves. Captain Falcon comes from a racing game. Peach is princess whom Mario rescues and only got playable roles after inclusion (and you can hardly call Mario a fighter, for that matter). Wii Fit Trainer, Villager, Mr. Game And Watch (who isn't even a real character) are also non-fighters.
You cant site Smash history as not giving a chance to non-fighters when it clearly does.
Anyway, besides Robin, I have a gripe about a lack of variety of body shapes and sizes among the newcomers. Too many of them would be plain, anatomically correct humans: WFT, Mac (besides height), Palutena, Isaac, Robin, Shulk... Compare that to the diversity of the Smash's original cast of twelve and you might find the lack of variety in this rumor suspicious.
After a certain point there isn't really much new that can be done in terms of body shapes.
Despite all this, I don't think this rumor is true. You don't need to look for a reason to deny a rumor. Rumors are rumors and shouldn't be considered real.