Harold was an acquaintance of Jack Harkness. He preferred to stay incognito and never gave Jack his real name.
Harold was of a humanoid species, but unnaturally pale and thin, with no hair, grey, slitted eyes and dark green tissue. He wore fancy, old-fashioned clothing, including a long black jacket, a top hat and black gloves. He was also a smoker. Like Jack, he worked as a conman.
At some point, Harold learned that the Hokrala Corp was planning to kill Jack Harkness and then destroy Earth. Using their warp shunt technology to travel back in time to 2009, he tried to warn Jack about their assassination attempt. He found out that he too was targeted by the Hokrala Corp assassins. He tried to transmat directly into the Torchwood Three to contact Jack, but this set off the defence systems. Jack, assuming him to be the assassin, shot Harold. Fortunately, Harold had recorded a Betamax cassette with all the information he had gathered, which Jack watched. (PROSE: The Undertaker's Gift)