Sylvleth

Sylvleth 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 pick the Blue Lions house, he will instantly be Byleth class. However, if Byleth chooses the Golden Deer house or the Black Eagle house he won't 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 an A level support. In their first support, Byleth sees Sylvain breaking up with a girl and blames him for breaking the girl's heart and for being a casanova. Then in their second support, he explains to them that the girls that are whith him for his crest, and not for he himself, and that they don't truly love him but want to gain nobility through a family agreement. He also expresses his hate for how his crest influences his life, and also mentions his jealousy of Byleth because they had grown up without knowing they had a crest. He later says it was a joke and that dark and brooding nobles "attract ladies". Their third support takes place after a battle where Sylvain was injured protecting Byleth. He says he is glad he did it despite the fact he was jealous of them, and he talks about his brother's hate for him, which Byleth sympathizes with. He then express that Byleth showed him another way to live and that he's glad to have met them. If the player chooses female Byleth in the gameplay, then the two of them can marry before the final boss. And if the player gets A level support with Sylvain, Byleth will visit their father's grave and take out the ring that he gave them. As the words of their father echoes, telling them to give the ring to a person that Byleth loves as much as Jeralt loved his wife, if the player chooses Sylvain after the battle, the two will reunite at the Godess tower, he will propose to marry them and they will accept. The two of them will wed after the war.

Azure Moon
If Byleth choses the Blue Lions, Sylvain will be one of their student since the beginning. After the time skip, they will reunite along the other 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 achieves a S support with him, they will marry.

Crimson Flower
On Crimson Flower, Byleth sides with Edelgard and the empire.

If Sylvain doesn't get recruited before the time skip, he will fight against the empire alongside his king, Dimitri, at Tailtean Plains. If female Byleth battles him, he expresses regret for not trying to woo her five years ago. He will die during the battle.

If he gets recruited before the time skip, Sylvain will abandon House Gautier, accepting a commission to be an Imperial Army general, believing in Byleth more than he does in his own country. After the war he becomes the new leader of the House Gautier. If Byleth marries him, they will help him rebuild the northern Fódlan. After this, they will work to end the differentiation of treatment between the person with a crest and those without one, to later have children.

Verdant Wind/Silver Snow
If Sylvain doesn't get recruited before the time skip, he won't appear in the Silver Snow route, and he will fight at the Second Battle of Gronder Field against the empire and the alliance (Byleth's side) in the Verdant Wind route. If he survives, during the battle, his fate is left unknown and probably dies after the fight.

If he gets recruited before the time skip, he joins the monastery and fights alongside Byleth and the church (and the alliance in Verdant Wind) against the empire. After the war Sylvain becomes the new Margrave Gautier and Byleth becomes the ruler of United Kingdom of Fódlan. If they get married, they also have children.

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 doesn't get chosen). 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. Some fanarts represent them in their summer alts from Fire Emblem Heroes. Many fanfictions take place after the game and describe their life with their family. Some of them explore Sylvain traumas.

Fandom
FAN FICTION

TUMBLR

Pixiv

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.