Robin, the vanilla, wrong representation of Fire Emblem: Awakening's main character, is a bit of a sneak in Smash Bros. First it's the flirtation with dark magic, now it's word that Robin's Nosferatu attack will damage enemies while reducing your damage -- and do so better from behind. Sakurai explained on the Miiverse:
"Robin's Nosferatu is the first ability in the Smash Bros. series that recovers your damage as it gives damage to others. The attack becomes even stronger if you strike from behind! Robin doesn't actually use this ability in the original game, but we added that element to make the character unique. Who knows--the ideas we use in this game may contribute to the original series."
It's going to be hard passing on the 3DS version and waiting for the Wii U.