In the novel The Mystery of the Blue Train, Johnnie Harrison is the youngest son of Dr and Mrs Harrison in St. Mary Mead. He does not appear in person, but is mentioned.
When Mrs Harrison visits Katherine Grey, Katherine says that she is glad that Mrs Harrison came, because she had been wanting to ask about Johnnie. Mrs Harrison then begins telling her a long story, "in which Johnnie's adenoids and tonsils bulked largely".