Sylvleth

Sylvileth is the non-binary ship between Byleth Eisner and Sylvain Jose Gautier from the Fire Emblem fandom.

Canon
Byleth meets Sylvain at the officers academy at the monastery, if they picks the Blue Lions house, he will instantly be Byleth class. However if Byleth chooses the Golden Deer house or the Black Eagle house he wont be in their class and will face them at the end of the first month, then Byleth can still recruit him, before the 5 year time-skip. The two can reach a A level support. If Byleth is female, then, they can marry, before the final boss, if the player gets A level support with Sylvain, Byleth will visit her father's grave and take out the ring that he gave her, his words will echo, of her farther telling her to give the ring to a person that Byleth loved as much as Jeralt loved his wife, if the player chooses Sylvain after the battle the two will reunited at the godess tower, he will propose to marry her and she will accept. They will wed after the war.

Azure Moon
If Byleth chose the Blue Lions Sylvain will be one of their student since the begening. After the time skip they reunite along the others Blue Lions at the monastery. They will fight together for the kingdom. After the war Sylvain becomes the new Margrave of Gautier and Byleth becomes the new Archbishop if Byleth archive a S support with him they will marry.

Fanon
The most popular version of this ship is the variant with female Byleth. This paring is quite popular since Sylvain is available in all routes and can be recruited in the early game if Byleth is female (even if the Blue Lions wasnt choosen). Due to the fact that in their paired ending (in non-Azure Moon routes) it's said that they have children many fanarts show them with their kids.

A popular joke in the fire emblem three houses fandom is that outside the Azure Moon route, Sylvain betrayed his family, kingdom and friends for a girl.

The version of this ship with male Byleth exist but is rare.

Fandom
FAN FICTION

Sylvain Jose Gautier/Byleth tag on A03

Trivia

 * If Byleth didn't chose the Blue Lions house and is female Sylvain will immediately accept a transfer to her class from as early as Chapter 2, regardless of her current stats.