Byleth is the child of Jeralt Eisner, a former knight of the Church of Seiros. Byleth does not recall much of their past. Jeralt has told them very little; however, following a chance encounter where they 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. 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.
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. They were released on January 28, 2020. Like his fellow Fire Emblem representatives Robin and Corrin, Byleth's female counterpart is also playable as an alternate costume.
- With a 2019 debut, Byleth is currently the most recent fighter to ever appear in the Super Smash Bros. series.
- Byleth is the first playable character whose console of origin is the Nintendo Switch.
- Byleth is the first fighter announced in the Fighter Pass not to be a third party character.
- Byleth is the eighth Fire Emblem representative in the Super Smash Bros. series, after Marth, Roy, Ike, Robin, Lucina, Corrin and Chrom.
- Byleth and Robin are the only Fire Emblem characters not to have a counterattack down special. In Byleth's case, they utilize Aymr.
- When KO'd, male Byleth may shout "Not my best!", while female Byleth may say, "Forgive me!", being one of the few characters to say something while KO'd.
- When Star KO'd, Byleth screams, "My battle ends here!", being the second character to speak their line for their Star KO. Pit is the first fighter to do so.