He cares for his friends and is admired as an older brother figure in his class. He is a perpetual skirt-chaser and often attempts to woo women with sweet words. He is fond of women but dislikes jealousy.
Despite the impression he gives off to people he's actually quite intelligent, being shown to easily grasp complicated magic formulas at a glance when magic isn't even one of his areas of expertise. In his supports with Annette it is seen that he intentionally leads people to think he isn't very smart because he dislikes the pressure of people's expectations and them believing they know what he's capable of, saying he finds it suffocating.
Sylvain appears to hold a strong level of disdain for the way Crests influence the lives of those in Fódlan, with several of his supports with other characters, and commentary in missions/paralogues, focusing on how they negatively impact peoples lives. In his supports with Byleth it is seen that his apparent easygoing, womanizing behaviour stems from the fact that he believes most women just want him for his bloodline, and not for who he is as a person. This is likely due to his Crestless brother, Miklan, being disinherited in favor of Sylvain, an heir with a Crest, and the Gautier family's strict codes regarding Crests. Sylvain says he's always been treated like he was special, presumably due to his Crest, though he himself doesn't think of himself as being different. He also ponders often how his life would have been if Miklan had inherited the crest instead of him.