Siobhan is one of Christopher's teachers at school. He seems really fond of her. Although Christopher's father understands him well, Siobhan is the only one who can really speak his language.
Christopher often mentions Siobhan when describing the aspects of human communication with which he has trouble. We assume it's through her that he's at least learned how to be polite and found out about this strange way of talking called "chatting." But our favorite Siobhan moment is when she reads Christopher's detecting book and offers to talk to him if he's feeling sad about what he's learning about his mother.
She probably gets the Sweet Award in this book, and we sure wish she were our teacher. Sigh.