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.

Fanon
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.

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.