Harold is a helicopter who patrols the Island, searching for emergencies.
When Harold first met Percy, he stated that railways were "slow and out-of-date", but was proven wrong when Percy beat him in a race. When Thomas' brakes failed while boarding at a station, with an inspector Harold chased him down the line. Christmas that year Harold brought Santa Claus to the party at Tidmouth Station. Harold also dropped hot drinks for Percy's crew when they were stranded in a floating river and made friends with Duck after rescuing an injured sailor.
As of Misty Island Rescue, he has joined the Sodor Search and Rescue team and lives in his own hangar at the Sodor Search and Rescue Centre. Since joining the team he has been involved in a search for Thomas when he got lost, saving Bertie from the Shake Shake Bridge and helping Charlie to recapture an elephant which had escaped from the Sodor Animal Park. Once, Harold's rotors became jammed, so Thomas and Duck took him to the Sodor Search and Rescue Centre.
He and the rest of the rescue team helped out after a big storm, and they were rewarded with a new alarm bell for the rescue centre. He later found Percy and Diesel trapped in a flood at the old quarry, and the rescue team helped them out. He also helped The Duke and Duchess of Boxford get to Callan Castle, after Thomas, Spencer and Bertie failed.
Harold speaks in a manner reminiscent of an RAF pilot, using terms such as "wizard" and "old chap" in conversation.
Throughout the second to fourth seasons, Harold somewhat tended to be full of himself as he felt that "railways are slow", but he always got his comeuppance in the end. He temporarily regained this part of his personality in A Bad Day for Harold the Helicopter.
Harold is based on a Sikorsky S-55 helicopter, built in the UK as the Westland Whirlwind. He has added floats.
Harold is painted white with red lining. His name is painted near his tail in red.