Lucina is the daughter of Chrom from the future and has the mark of Naga, known as the Brand of the Exalt, on her left eye. In order to hide her identity (but still help save the world along with other travelers) Lucina changed her hair, donned a mask from Gerome to hide her Brand of the Exalt, and took up the namesake of her ancestor, Marth. When she journeyed to the past, Lucina came to Chrom and Lissa's aid when a Risen nearly killed Lissa. In Chapter 6, her identity was discovered when she saved Chrom from an assassin. In Chapter 14 she joined her father and the Shepherds on their quest to Valm and to stop the future doom from ever happening.
Super Smash Bros.
Introduced in Super Smash Bros. for Nintendo 3DS / Wii U, she is a clone of Marth and thus shares the same moveset. The only different property is that the attacks have a uniform hitbox throughout the blade instead of only at the tip like Marth's.