Byleth (♂: ベレト, ♀: ベレス, Byleth) is the default name for the player character in Fire Emblem: Three Houses. In the game, Byleth assumes the role of a professor to one of the three titular houses at Garreg Mach Monastery, guiding the students through day-to-day activities and improving their combat skills.
Byleth Eisner is the main character of Fire Emblem: Three Houses, following in the footsteps of Corrin in also serving as a player surrogate. Unlike past player avatars Robin and Corrin, however, the only customizable features of Byleth are name, gender, and birth date, and they are portrayed as a silent protagonist, with the player choosing Byleth's dialogue options from a menu as they interact with other characters.
In Three Houses, Byleth is the child of Jeralt Eisner, a former knight of the Church of Seiros. Byleth does not recall much of their past and Jeralt has told them very little; however, following a chance encounter where Byleth and Jeralt rescue Edelgard von Hresvelg, Dimitri Alexandre Blaiddyd, and Claude von Riegan from a group of bandits, they are brought back to Garreg Mach Monastery, where Jeralt is reinstated as the captain of the Knights of Seiros and Byleth is enlisted as a professor for one of the Officers' Academy's three houses: the Black Eagles led by Edelgard, the Blue Lions led by Dimitri, and the Golden Deer led by Claude. Depending on the player's decision, the story plays out differently.
Sothis is a mysterious entity who appears in Byleth's dreams and, eventually, manifests in real life, although Byleth is the only one that can see her. She has lost all of her memories except her name, but possesses great power, bestowing Byleth the ability to turn back time. Eventually, after Byleth is banished to a dark realm by a mysterious group called Those Who Slither in the Dark, Sothis rediscovers her past and learns that she is a goddess; she merges fully with Byleth, thus granting them all of her power but ceasing to appear to them after that point, except in a few select circumstances. Sothis is also the source of Byleth's Crest, the Crest of Flames, which allows them to wield the Sword of the Creator.
Regardless of what house they choose, at the end of Byleth's first year as a professor, Edelgard declares war on the Church of Seiros and Byleth is forced to take a side. An accident results in them falling into a coma for five years; once they awaken, they reunite with their house of choice (or the Church of Seiros, should they decide to side against Edelgard after choosing the Black Eagles) and assist their house leader in their goals: Edelgard to destroy the Church, Dimitri to stop Edelgard, Claude to discover the truth behind Crests and the Heroes' Relics, and Rhea to stop Edelgard and Those Who Slither in the Dark.
As a playable character
Main article: Byleth (SSBU)
Byleth was announced as a DLC fighter in the Nintendo Direct broadcast on January 16, 2020. Both male and female versions are available and are the fifth DLC character available through the Fighters Pass Vol. 1. They were released on January 28th, 2020 as part of Challenger Pack 5.
According to a February 2020 Famitsu interview, Masahiro Sakurai reiterated that Byleth, like the other characters in the Fighters Pass, was a choice made by Nintendo, and the theme for his inclusion was a character from a very recently released game. Unlike previous DLC fighters, which could borrow their moves and animations from legacy titles, Byleth hailed from a game that was still under development. He borrowed and played through a development ROM of Fire Emblem: Three Houses and realized the game's complex nature, as the plot branched off into three different stories. Finally to remedy potential criticisms of an overabundance of sword-users, it was decided that Byleth have the Hero's Relics from the three house leaders as part of his moveset.