Sylvain is a member of the Blue Lions and a heir to a noble house in the Holy Kingdom of Faerghus. Somehow I got him in my party, without me trying. He's good with a lance, but in the story he's a shameless flirt, always out to break some poor girl's heart.
The more serious side to him is his older brother Miklan. Unlike Sylvain, he wasn't born with a magical crest and was casted out from his family. He wages war against Byleth and the Church of Seiros or whatever.
And his older brother becomes a demonic monster who served as my first real challenge of the game.
Post-time skip, Sylvain didn't change much. Byleth always had to give him a bad look after some girl ran away from Sylvain crying. He was a decent fighter in my party, but too much like Ferdinand for my tastes.