Marth (マルス, Marth) is the main protagonist of four games in the Fire Emblem series of tactical role-playing games. He debuted in the first and third games, Fire Emblem: Shadow Dragon and the Blade of Light and Fire Emblem: Mystery of the Emblem, and later reprised his role in the Nintendo DS remakes of these two games. He made his first appearance in the Super Smash Bros. series as a playable character in Super Smash Bros. Melee. At the time, no Fire Emblem games had ever been released outside Japan, but his popularity in Melee (along with that of his counterpart Roy) is believed to have influenced Nintendo's decision to release almost all subsequent Fire Emblem titles internationally.
Marth was introduced in the first Fire Emblem game, Fire Emblem: Shadow Dragon and the Blade of Light for the Famicom console, which was later remade for Nintendo DS as Fire Emblem: Shadow Dragon. While he was absent from the tangentially-related second game in the series, Fire Emblem Gaiden, Marth's role and place in the story of Archanea was later expanded on in the third game, Fire Emblem: Mystery of the Emblem, released on the Super Famicom in early 1994. Mystery of the Emblem featured both a remake of the original game and a continuation of that game's story, and the continued story was itself later remade for Nintendo DS as Fire Emblem: New Mystery of the Emblem, Heroes of Light and Shadow.
Marth is the prince of Altea, one of the Seven Kingdoms of the continent of Archanea. He is descended from the family of a legendary dragon-slayer, Anri, who once used the "blade of light", the Exalted Falchion, to defeat the sinister dragon-emperor Medeus. Marth is characterized as a courageous, good-natured and honorable man. He takes his royal responsibilities to his people very seriously, always putting their needs first, no matter how troubled he is by developments in his own personal life. As a leader, he not only cares deeply about his friends and comrades but believes that he could not have achieved anything himself without their support. Later appearances tend to emphasize Marth's determination to save as many people as possible and ensure that none of his comrades die, even in seemingly hopeless situations.
In Marth's first adventure, told in Shadow Dragon and the Blade of Light and its remake, Medeus has been revived by the sorcerer Gharnef and has conquered almost all of Archanea, and by the time that the games begin Marth has spent two years in exile in the remote island kingdom of Talys with his future wife, Princess Caeda. The games follow his quest in the War of Shadows, where he works with Princess Nyna of the Holy Kingdom of Archanea to unite the defeated human kingdoms into an army to fight back against Medeus and his dragon empire, Dolhr, and gain the power to fight Gharnef so that he can retrieve Falchion and rescue his abducted sister, Elice.
The second story in Mystery of the Emblem begins three years after the events of the first game, and follows Marth as he is confronted by his former friend and ally, Hardin, now the Emperor of Archanea and corrupted by Gharnef. After Hardin conquers Altea and declares Marth a traitor, Marth and his allies embark on a journey through Archanea's northern wilderness and follow the legendary path once taken by Anri, where he learns the true history of the dragons and their role in shaping the history of Archanea. Marth is given the task of repairing the titular Fire Emblem shield to restore its ability to seal away the earth dragons, a malevolent species of a dragon to which Medeus belongs. The game ends with Marth uniting all seven of Archanea's kingdoms into one after Medeus's final defeat and becoming revered as the legendary "Hero-King".
In his starring games, Marth is given the unique lord character class: as the commander of his army, it is necessary for him to seize castles and thrones held by enemy armies to complete chapters and progress in his quest, but if he is defeated in battle, the game is over and the player must restart from their last save point. As the first lord in the Fire Emblem series, Marth established many trends in both narrative and gameplay roles that later series protagonists, such as Roy and Chrom, would closely follow. The majority of Fire Emblem protagonists share the lord class with Marth, with only a few notable exceptions, such as Ike and Corrin. Despite only being the star of four out of the series' sixteen games, Marth is treated by Intelligent Systems as the mascot of the entire Fire Emblem series.
Outside of his own games, Marth is also a notable part of the backstory of Fire Emblem: Awakening, and two of its protagonists, Chrom and Lucina, are his distant descendants. He has made frequent appearances as an optional bonus character, unrelated to the main story, in many Fire Emblem games from Awakening onward; he is available in Awakening via SpotPass and DLC, Fire Emblem Fates via amiibo and DLC, and Fire Emblem Echoes: Shadows of Valentia via amiibo as an illusory unit. He features prominently in Fire Emblem crossover productions, including the video games Fire Emblem Heroes, and Fire Emblem Warriors as one of their many playable characters, and the series' two trading card games. He was also the star of a short-lived direct-to-video anime adaptation of Mystery of the Emblem in 1997, which featured actor Hikaru Midorikawa's first performance as the character, five years before he reprised that role in Super Smash Bros. Melee. In the OVA, Marth's English voice was provided by Spike Spencer, albeit erroneously localizing his name as "Mars". He did not receive another English speaking role again for 18 years, until the 2015 release of Code Name: S.T.E.A.M. for the Nintendo 3DS. From this point on, including Super Smash Bros. Ultimate, Marth would be fully voiced in English by Yuri Lowenthal.
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. 
As a playable character
Main article: Marth (SSBM)
Marth makes his Smash debut (and by extension, his international 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 on 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 or failing 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 most 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. Marth's "clone", Roy, has attacks that are almost identical in animation, 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.
Main article: List of SSBM trophies (Fire Emblem series)
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:
As a playable character
Main article: Marth (SSBB)
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 a superior reach to most characters in the game and relies on the length of his sword to effectively zone against his opponents. While his changes are mixed, he was somewhat 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.
Main article: List of SSBB trophies (Fire Emblem series)
As a playable character
Main article: Marth (SSB4)
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 design from Fire Emblem: New Mystery of the Emblem: Heroes of Light and Shadow, although Falchion retains its design from the original Fire Emblem: Mystery of the Emblem.
During the initial release of SSB4, Marth was considerably nerfed, with few noteworthy buffs. These nerfs left him with less mobility and range relative to the cast (though his own range had been improved from Brawl), weakening his neutral game and depriving him of his potent comboing and punishing capabilities from previous games. Because of this, he had previously seen a huge drop in his tier placement. However, he has since received multiple buffs in update patches, with the most notable patch, 1.1.4, granting him improvements to his aerial combos and juggling game, as well as granting bigger tipper hitboxes, and higher damage output on several attacks. As a result, Marth is reasonably better than he was during the initial release of SSB4, and has since seen significantly improved tournament results. Overall, although he's still considered fairly nerfed from Brawl, he ranks highly on the SSB4 tier list, ranking 11th out of 55 characters (tying with Ryu).
As a playable character
Main article: Marth (SSBU)
Marth returns as a playable character in Super Smash Bros. Ultimate, as with all previous characters. He uses a more detailed version of the New Mystery of the Emblem design from SSB4, and now has a different hairstyle to match his more recent appearances in titles such as Fire Emblem Heroes. Like in Melee and Brawl, Marth is an unlockable character. Ultimate is the first Super Smash Bros. game in which Marth is dubbed in English in overseas versions, specifically by Yuri Lowenthal, his English voice actor in all appearances from Code Name: S.T.E.A.M. onwards. In Japanese, Hikaru Midorikawa returns to voice Marth with new voice clips for the first time since Brawl.
As Marth was a high or top tier character in all of his previous appearances, he received a mix of buffs and nerfs in the transition from Smash 4 to Ultimate, but was nerfed overall. While he received buffs both directly and from some universal changes, the faster pace of Ultimate combined with the removal of micro spacing techniques like perfect pivoting makes it much more difficult to land his tipper sweetspots. He also received several direct nerfs: his jab is no longer an effective combo starter, and his sourspots are much weaker. As a result of these nerfs, Marth sees little representation in tournaments, with many players considering him outclassed by Lucina.