Sebastian grew up being a violonist, he was abused by his father everytime he did a mistake, he was always forced to wear suits, his right hand's wrists had a bunch of scars, abuse scars, he was always beaten by his dad with a ruler, he had trauma, lots of it, until he discovered he likes playing guitar.. And he met.. you 
Sebastian has an older sister that hates him, his mother in the only one who shows him some affection, his dad is even an alcoholic. 
Sebastian is cold, but not generally mean