Harry Middleton was a friend of Bruttenholm and Hellboy. According to Abe Sapien, he went to Eton College with Bruttenholm. While attending Eton the pair encountered their first ghost near a Scottish monastery. He worked with them in the 1950's on a number of cases including 'The Phantom Hand at St. Albans,' and 'The Braunton Burrows Hound'.
Following Hellboy's time at sea (as seen in The Island), he stayed with Harry Middleton for one month, before being taken to a gathering of witches (as seen in Darkness Calls). Harry, along with the ghosts of Sir Albert Lindsay and Lady Celia Radford, played cards with Hellboy two weeks before the events of Darkness Calls. Harry also gave Hellboy a 1941 U.S. Army .45 in a left-handed holster, to replace Hellboy's lost gun.