Sebastian or Seb Leroy is the son of Morton Leroy, who is a seriously dangerous man. We first meet Seb, aged about 14, as one of the youngest guests at the funeral in the great house next to Granny's home. He is wearing a rather posh suit and looking smooth and superior. In many ways, he seems to be a sort of distant twin of Draco Malfoy.
Later in the novel, protagonist Polly Whittacker meets him, and somehow impresses him as a good listener -- someone he can talk to about the pop groups he's currently interested in. Seb goes to the expensive private school in the same town as Polly's very working class comprehensive school. In the end, even though she pities him, Polly is not drawn to him as a friend at all.