The same, but different
Lucina, originally introduced to us as Marth in Fire Emblem: Awakening, will in fact share his move set in the upcoming Smash Bros. Masahiro Sakurai explained on the Miiverse:
"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."
In the same way characters like Falco and Fox are split, so too are Lucina and 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."