The fun loving, motorcycle riding friend of Kelly.
In Season of the Witch, she first appears when she comes to the rescue of the stranded Kelly and Mr. Wilkes. Kelly and Sophie's friendship builds from there, sadly its all a lie. What Sophie's true motive is has yet to be revealed but what has been revealed is that Sophie has been manipulating events---and people---for her own selfish means.
She is the only person who actually saw Kelly for the boy he was rather than mistaking him for a girl like everyone else. When she meets girl Kelly she believes male Kelly is crossdressing.
In Ch.23 of Season of the Witch, its revealed that Sophie is in fact a witch herself---one who has a dangerous ability to control people's mind and who has a perverse interest in Kelly. In Ch.24, she's placed in a binding collar to stop her powers but at the end of the chapter it is shown that she has escaped.
Sophie's current whereabouts are unknown.