Peter Calloway works the reference desk at Buxton Library. He is referenced in Cambion only once, in a single sentence in Chapter Thirty-Eight: The Null Hypothesis. He does not appear in any scene. He has no spoken lines. The reader knows him entirely as the reason Daniel Marsden takes three buses to Marple instead.
The single canon line: He had avoided Buxton Library; he’d already been in twice this term — once in the first week back, once the week before, and Mr Calloway on the reference desk knew his face. The sentence is the entire architecture of Calloway’s presence in the saga. He is the institutional figure whose ordinary, unconscious habit of recognising regular visitors is, for Daniel Marsden, an operational hazard. Daniel is researching things he does not wish to be observed researching. Calloway has noticed him twice. That is the whole problem.
Calloway is not aware that he is anyone’s problem. As far as he knows, he is the reference librarian at Buxton Library, and a particular boy has been in twice this term, and that is unremarkable.