Who is Harold Ferrell from The Egyptologist and what is their importance?

Harold Ferrell is an Australian detective hired by Barnabas Davies to track down Paul Caldwell, one of Davies' illegitimate children. Ferrell finds Paul's mother in Sydney, along with Paul's connections to a circus and a librarian. His relationship with the librarian, Catherine Barry, leads Paul to develop a keen interest in ancient Egypt. When Catherine rejects Paul's overture of romance, he joins the army and serves during World War I in Egypt. Ferrell discovers that Paul and Hugo Marlowe, an officer in the British army, go missing just after the war ends, and are presumed dead.