Board Thread:Super Smash Bros. Discussion/@comment-32390199-20140707015357/@comment-25192437-20140727045851

Yorky97 wrote: I don't think Lucina ever started out as an alternate costume. Sakurai himself stated in an article in Famitsu that Lucina started out as an alternate costume for Marth, then developed into her own character as a result of them slightly altering how she plays. He was thinking about adding Chrom, but decided against it because there was really nothing to truly differentiate him from Marth or Ike and Smash had enough men wielding swords.

http://www,gonintendo,com/s/233268-sakurai-talks-lucina-s-inclusion-in-smash-bros-wii-u-3ds-why-chrom-didn-t-make-the-cut (replace the commas with periods)