Lucina

Lucina is the future daughter of Chrom who made her debut as one of the main characters in Fire Emblem Awakening and appears as a newcomer playable character in Super Smash Bros. for Nintendo 3DS / Wii U, revealed in the same trailer as Robin, another character from Fire Emblem Awakening. In her game of origin, she initially appears disguised as a masked "Marth", having traveled through time to stop the awakening of the evil Fell Dragon Grima, helping Chrom's Shepherds on occasion and eventually joining them after revealing her identity. Lucina wields the Parallel Falchion, a reforged version of the same sword used by Marth and Chrom.

Super Smash Bros. for Nintendo 3DS / Wii U
Lucina is an unlockable character in both Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U. To unlock her in the Nintendo 3DS version, the player must either complete Classic Mode as Marth without the use of continues or play 40 Smash matches, then defeat her in a 1-stock match on the Arena Ferox stage. In the Wii U version, the player can unlock her by completing Classic Mode on Intensity 5.5 or higher or by playing 30 Smash matches, then defeating her in a 1-stock match on the Coliseum stage. She is voiced by Yū Kobayashi in the Japanese versions of the games, and by Laura Bailey in the English versions, both reprising their roles from Fire Emblem Awakening. She was originally meant to be an alternative costume for Marth, but was later made a separate character, the reason being that she has traits that differentiate her from Marth. Lucina is the only unlockable character that was announced before the Japanese release of the Nintendo 3DS version.

Lucina's moves are all shared with Marth, including her custom variants: her standard special is Shield Breaker, her up special is Dolphin Slash, her side special is Dancing Blade, her down special is Counter, and her Final Smash is Critical Hit. However, the main difference between Marth and Lucina is in their sword attacks; while Marth's sword delivers more damage near the tip of the blade but is otherwise extremely weak, Lucina's is balanced throughout. Lucina's stage entrance involves her warping onto the stage from a magic circle, removing her mask in the process.

Lucina's palette swaps are based on other female characters in Fire Emblem Awakening. In addition, one of her taunts has her briefly put on the mask she wears when disguised as "Marth", which is also used by Kirby when she is copied by him. Finally, Lucina has alternate victory quotes that have a chance of playing if she defeats either Marth or Ike.

Super Smash Bros. Ultimate
Lucina returns as an unlockable playable character in Super Smash Bros. Ultimate. This time, she is classified as an Echo Fighter of Marth. Lucina's victory theme is now a short arrangement of "Id (Purpose)", a song from Fire Emblem Awakening. Lucina retains the same major gameplay difference setting her apart from Marth; her sword attacks dealing consistent damage across the entirety of her blade.

Classic Mode route
Similarly to Young Link's route, Lucina's Classic Mode route features other Fire Emblem fighters; like with Pac-Man, the opponents are fought in backward chronological order of their games of origin were released.

Palutena's Guidance

 * Pit: That lady's moves are awfully similar to Marth's.
 * Palutena: That's Lucina. She used to introduce herself as Marth, so certain similarities seem inevitable.
 * Pit: What? She stole someone else's identity?
 * Viridi: Well, it wasn't quite like that.
 * Palutena: Marth had become a legend by her time—the Hero King. So Lucina took that name to bolster the morale of her own army. Think of it as a symbol of hope in the depths of despair.
 * Pit: That's pretty deep.
 * Viridi: You can't tell by looking at her, but I hear she's got a great sense of humor.
 * Pit: I'll try cracking a few jokes and see if that softens her up.
 * Palutena: The tip of her blade doesn't deal as much damage as Marth's. But her attacks are more balanced, even if they're not tremendously powerful.

amiibo

 * Lucina, from the Fire Emblem Awakening game, is Chrom's daughter and the future Princess of Ylisse. When she arrives from a doomed future seeking to prevent it from ever happening, she goes by the name Marth before her identity is revealed. She loves her father and would do anything to keep him safe.

Super Smash Bros. Ultimate Super Smash Blog bio

 * Lucina is Marth's echo fighter, so she shares most of her standard and special attacks with Marth. However, while Marth's attacks are more powerful when made with the tip of his blade, Lucina's attacks just as powerful whether you use the tip or the base of the sword.

Quotes

 * "You're mine!" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "My turn!" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "Come at me!" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "I cannot lose." - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "Father, I won." - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "And they call you the Radiant Hero?" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "The future is not written!" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "You'll never defeat me!" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "This is the Hero-King?" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U
 * "Konna tokoro de...! (Must I fall here...?)" - Super Smash Bros. for Nintendo 3DS / Super Smash Bros. for Wii U (Japanese version only)

Trivia

 * Like with her official appearance, Lucina's in-game model features the Mark of Naga on her left eye. Masahiro Sakurai has said that, as it is such a small detail, most players likely will not notice it during normal gameplay.
 * Along with King Dedede, Lucina's name is announced differently in the Japanese and Korean versions of the Super Smash Bros. games to reflect the pronunciation in said languages. As of Super Smash Bros. Ultimate, this also applies to Simon, Richter, and King K. Rool in the Japanese version.