James Stevens
James Stevens, authored Who Killed Kennedy, a book chronicling his attempts to learn more of the secretive organisation known as UNIT.
Biography
Stevens discovered, and found himself obsessed by, UNIT, "Dr. John Smith" and a history of alien intervention in Human affairs dating back at least to the Shoreditch Incident. He met Dodo Chaplet, a mentally broken one-time companion of the Doctor in first incarnation as an informant. However, he developed a romantic relationship until, forces determined to silence her, murdered her. At her funeral, Stevens briefly met the Doctor himself.
Having traveled to Dealey Plaza in 1963 to witness the Kennedy assassination, Stevens had seen his own older self shoot Kennedy. He remained haunted by the knowledge, that, to ensure that history kept to its proper course, Stevens knew that he must eventually return to that time and place to kill Kennedy. (MA: Who Killed Kennedy)
Before leaving the present to commit the assassination of Kennedy, Stevens would mentor Ruby Duvall, a young aspiring journalist. (NA: Happy Endings)
Stevens disappeared sometime in 1995, presumably having time-travelled into the past. (whoisdoctorwho.co.uk)
Behind the Scenes
Who Killed Kennedy came out with the fictional James Stevens listed as its co-author, with its real author, David Bishop. Stevens supposedly appears in the form of one of the unnamed journalists who appear in Spearhead from Space and played by various extras.
