Warning: The following page/section contains spoilers. Please read at your own risk.
Byleth was born as the child of Jeralt and Sitri Eisner on the 20th of the Horsebow Moon in Imperial Year 1159. Sitri died after giving birth under mysterious circumstances unknown to most, leaving Jeralt devastated as he was told she died from childbirth complications. Jeralt called a doctor in secret to examine Byleth, only to learn that Byleth had a pulse-- but no heartbeat.
In truth, Sitri was the 12th vessel created by Rhea to try and revive the Goddess Sothis. Sitri, along with the predecessor vessels, were unable to host the power of the Goddess, which made Sitri frail and unable to leave the monastery. When she met and fell in love with Jeralt, Jeralt had the Crest of Seiros, creating a perfect recipe for reviving the Goddess in their child. Byleth and Sitri were about to die, where Sitri demanded Rhea to transfer her heart into Byleth's, where Rhea agreed. While the transplant was successful, it killed Sitri and made Byleth lack heartbeat.
Jeralt's suspicions of Rhea grew after the events, making him think she did something to the child. He then thought of a plan to leave the Church of Seiros along with the knights suddenly.
On the 7th of the Wyvern Moon in 1159, Jeralt left the Knights of Seiros to go to Remire Village to raise Byleth as a mercenary, using a fire that broke out to fake the baby's death.
Byleth was then raised as a mercenary, gaining the nickname Ashen Demon from others for striking down their enemies in combat without emotion or hesitation.
Byleth was announced as a DLC fighter for Super Smash Bros. Ultimate on January 16, 2020. Byleth's primary weapon is the Sword of the Creator, which is used in both sword and whip form for his upward attacks. They are also equipped with Areadbhar for their side moves, Aymr for their down moves, and Failnaught for their neutral moves. Byleth is mostly specialized in distance based combat and a lot of their moves are less effective when the opponent is too close. Byleth has alts referencing Dimitri, Edelgard, Claude, and Sothis, as well as their transformation state for both genders.
Male Byleth's original voice actor was Chris Niosi. However, due to Niosi confirming abuse of several ex-girlfriends, as well as being accused of breaking the non-disclosure agreement, he was recast and replaced by Zachary Aguilar.
Names in Other Languages
Spanish: Bereto (based on his Japanese name. Second declension (genitive Beretis))
Portuguese: Bereto (based on his Japanese name. Second declension (genitive Beretis))
Italian: Beréto (based on his Japanese name. Second declension (genitive Berétis))
Spanish: Beresu (based on her Japanese name. Fourth declension (genitive Berésuos))
Portuguese: Beresu (based on her Japanese name. Fourth declension (genitive Beresús))
Italian: Berésu (based on her Japanese name. Fourth declension (genitive Berésuus))