Marth (マルス, Marth) is the main playable hero in the first and third installments of the Fire Emblem series of tactical role-playing games. Previously well-known primarily in Japan for a game series that was never localized abroad, Marth was included in all language versions of Super Smash Bros. Melee as a playable character, and his popularity in the game (along with that of his similar counterpart Roy) influenced Nintendo's decision to release all subsequent Fire Emblem titles (excluding New Mystery of the Emblem) internationally.
The first game in what would become the long-running tactical role-playing medieval high fantasy series Fire Emblem, released on the Famicom (Japanese NES) and entitled Fire Emblem: Shadow Dragon and the Blade of Light), introduced Marth as the main character. His design was noticeably different from his modern-day incarnation. The second game in the series, Fire Emblem Gaiden, took place on the same fictional world but on a different continent and was only tangentially related to the first and did not feature Marth. The third game however, titled Fire Emblem: Mystery of the Emblem and released on Super Famicom (Super NES) early in 1994, was both a remake of the original game and a continuation of that game's story. Marth underwent a character redesign to resemble the more "bishōnen" prince he is portrayed as in today's modern image of him, and he became the only FE -series main character to feature in a starring role from the start of the game in more than one FE title. In both these games, his in-game unit is the only one that belongs to the Lord character class, and that became tradition with all main starring characters of subsequent FE titles, being units that must be kept alive and start out weak but become very powerful over the course of each game.
Marth also stars in remakes of the first and third Fire Emblem games on DS. He is also a notable part of the backstory of Fire Emblem: Awakening, and two of its protagonists, Chrom and Lucina, are his distant descendants. Marth is available as a playable character in Awakening (via SpotPass and DLC) and Fire Emblem Fates (via amiibo and DLC).
Neither Marth nor any Fire Emblem content appears in the first Smash game. However, according to an interview from Making of Fire Emblem: 25 Years of Development Secrets, Masahiro Sakurai wanted to include Marth as a playable character in that game, but was unable to do so due to time constraints. 
Marth makes his Smash debut (and by extension, his international video game debut, excluding the Mystery of the Emblem anime adaption, which was released in North America) as an unlockable character in Melee. His design is based off his appearance from Fire Emblem: Mystery of the Emblem. He can be unlocked one of three ways: using all fourteen starter characters at least once each in Vs. Mode matches, beating Classic mode with all fourteen starter characters on any difficulty, or playing 400 Vs. mode matches. In each of these cases, after fulfilling the conditions Marth must be battled one-on-one to be unlocked.
Marth is one of the game's more effective fighters, currently ranking 3rd on the Melee tier list. His strengths include the long reach of his sword attacks and the very good damage and knockback specs of the "sweet spot" on the tip of his blade, his moderately fast dash, long grab, usefully floaty wavedash, excellent edgeguarding capabilities, easy and efficient combos, and great SHFFL, with his drawbacks being his lack of a projectile attack and laggy up special recovery. Marth's "clone", Roy, has attacks that are almost identical in appearance, and many "Marth vs. Roy" debates ensued in the years following Melee's release but professional analysis places Marth in a higher tier than Roy because he lacks Marth's subtle but vital advantages.
All of Marth's standard and special attacks involve him using his sword, the Falchion.
As with the other playable characters in Melee, Marth has three trophies which are obtained by defeating the single-player modes - a normal trophy from Classic Mode and two "Smash" trophies from Adventure and All-Star Modes respectively. The text of his Classic Mode trophy reads:
Marth returns as an unlockable playable character. Like Melee, his appearance is based on his design from Fire Emblem: Mystery of the Emblem. Marth has superior range to most characters in the game, and relies on the length of his sword to effectively zone against his opponents. It is uncertain as to whether or not he has been nerfed from Melee like a lot of other higher tier characters. He is currently in the A-tier according to the official SBR tier list.
Marth returns as a playable character in Super Smash Bros. for Nintendo 3DS / Wii U. Marth is now a starter character, as opposed to being an unlockable character as he was in Melee and Brawl. He has been visually updated to closely match his attire from Fire Emblem: New Mystery of the Emblem, Heroes of Light and Shadow as opposed to his design from Fire Emblem: Mystery of the Emblem, which was used in Melee and Brawl.
In SSB4, Marth was nerfed significantly, amidst some exiguous buffs. These nerfs leave him with less mobility and range, weakening his neutral game and depriving him of his potent comboing and punishing capabilities from previous games.