Ah, Yarne.
I was about to make this thread myself, so it's a good job I checked first...
"No. YOU GO EXTINCT!" Yarne for Smash 4!
Why Yarne?
Yarne isn't a sword with a lord. Far from it in fact! The last of the Taguel's would rather avoid danger at all possible costs.
In fact Yarne overall is a powerhouse, but incredibly timid. His sense of self preservation means that although he has great power he thinks cautiously and considers all possible dangers, fearing death itself for not only himself but also his team.
If that is not the very physical embodiment of a player playing Fire Emblem I don't know what is.
To the uninitiated death is a permanent thing in Fire Emblem. Once you're dead, you're dead. There are no 1Ups, there are no Fairies that can revive you, no phoenix downs. You're stone cold dead. This leads to much annoyance, fear and resetting amongst the FE fanbase. So why not represent that aspect of the series which is far more prevalent than even the weapons triangle, magic users and many other things?
Yarne is perfect for this.
What can Yarne do?
Yarne is a Taguel and as such can transform into a massive beastly wererabbit. His strength and speed are immense, but he's also a large character as a result of this sudden bulk up. Naturally as a rabbit beast speed and jumping prowess are second natures to him, as he fights with his claws and back feet.
By using the Beaststone Yarne can initiate the transformation into his mystical beast form(his default fighting form within the game). The beaststone transformation first covers his body in a large rocky shield(perfect for a custom shield).
Yarne as mentioned in the OP post DOES have access to swords, but frankly; don't we have enough at this point?
Personally I've always imagined Yarne to fight with the use of his Taguel transformation;
For example his down aerial could be him suddenly shifting into Taguel beast form and with the additional speed and power of the sudden increase of mass, kicks the opponent straight down before turning back.
More of an on the fly transformation ability than Sheik and Zelda, the Taguel form would be included within his moves instead of a singular transformation form making for two separate characters.
He'd have a lot of speed and sudden bursts of pure power, but many of his normal attacks would be weak to compensate, and he'd be quite lightweight and easy to KO.
Other options for Taguels
There is his mother Panne who is far more courageous than he is, being a more atypical stoic warrior type and officially the last of her kind(As Yarne is half human) but the reason I support Yarne more over her is that he represents and embodies the feel of Fire Emblem far greater than she does. She isn't afraid of death, despite being the last of her kind, hell she's not even in a rush to save her species by repopulation.
Yarne also has a more comedic personality and some of the greatest crit lines available "NO, YOU GO EXTINCT!" which is a definite must to accompany his Final Smash, whilst "Don't mess with a Bunny!" would make for a fine taunt or Victory speech.
Other options for representation of Fire Emblem on the technical scale:
Robin of course is not only more likely, more popular and capable of magic, but s/he is also able to represent Fire Emblem's tactics edge being the tactician. S/He is the one that keeps the group safe through strategy and is the only one that comes close to representing an aspect of Fire Emblem like this. On the downside for Robin; more swords and a far more customizable appearance within the games.
Of course Yarne isn't very likely to begin with, but he does have a set appearance(albeit one tuft of hair changes colour depending on his fathers hair colour) and also represents that same meta concept of trying to avoid death at every possible turn. He also doesn't wield swords as a primary weapon, nor is he even (completely) human showcasing some of the more varied creatures of the Fire Emblem world than Robin would be capable of.
Overall Yarne is a fun character with lots of moveset potential revolving around transforming into a monster on the fly, and his own naturally rabbit like abilities who represents a meta concept that has been present in every single Fire Emblem game.
