Along with Robin, Lucina joins the battle!! Her physical ablilites are identical to Marth’s–it must be in her D.N.A. However, where Marth’s power is concentrated in the tip of the sword, Lucina’s attack strength is balanced throughout the weapon, which might make her easier to control. She’s also a little bit shorter than Marth.
With the Wii Fit Trainers, the male and female Robins, the Villagers, and Little Mac, varying their appearances and voices works just like selecting alternate color variations. However, whenever there is even a small difference in abilities, that character gets an actual roster slot. That is why you can select Lucina individually.
By the way…there’s probably no chance you’ll be able to see this in the game, but Lucina has the mark of Naga in her left eye.