In the novel The Mirror Crack'd from Side to Side, Alfred Pike is the second son of Mrs Pike. Miss Marple says that he seemed "perfectly rational and normal", and almost "painfully prosaic", but he actually had the most abnormal psychology, to the point of being dangerous. He now resides in Fairways Mental Home, where he seems quite happy and contented. The staff of the home understand him, and the doctors consider him an interesting case, which pleases him vvery much.
Miss Marple mentions Alfred after she and Inspector Craddock discuss the possibility that someone slightly mentally unhinged might want to kill Marina Gregg, and Craddock says that such mental instability does not always show.