Seb is about four years older than Polly; when they first meet, he is about 14, about a foot taller than her, and strikes her as selfish, scornful, and rude (although given the occasion -- see Fire and Hemlock (spoilers) -- he has some excuse). At this age he likes the Who, the Rolling Stones, and David Bowie, and has discovered Michael Moorcock.
At 15, he has grown about a foot taller, has started smoking (despite his father's objections), and likes the Doors (though he is now sick of the other bands). He carries a silver lighter, wears a sheepskin jacket, and has a laugh which strikes Polly as "a sort of elegant churring."
At 16, he is tall and well-dressed and has a passion for photography; he owns a camera, which may be the same camera which Thomas Lynn once gave him. He is Head Boy at Wilton College, is planning to be a barrister, prefers to be called Sebastian, is studying for A-levels, and takes a picture of Polly which he puts up in his study. At 17, he rides a motorbike.
He evidently makes no secret of his romantic attachment to Polly, since she is "famous" at Wilton College as the object of Seb's devotion, but in general he maintains a self-possessed and slightly scornful demeanor. Although he applies to Oxford and Cambridge, he attends London University.
Fiona, who does not care for him, calls him Marmaduke.