When Harold first met Percy, he stated that railways were "slow and out-of-date", but was proven wrong when Percy beat him in a race.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. For 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 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 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.
In the twentieth season, he and Percy's rivalry resurfaced at Christmas. Percy was struggling to get through the snow while delivering mail for Santa, so Harold offered to help, but Percy refused. Percy then left behind a sack of letters at Dryaw and Harold decided to get them to him. Percy, believing Harold was mocking his speed again, decided to race him, only to crash into a snowdrift. Harold again offered to help deliver the mail and this time, Percy agreed. While he was able to deliver the mail to Hiro, he ran out of fuel and was grounded in a field on the Mainland for the night. When Percy found out, he raced off to find him and then brought him some fuel, not wanting him to spend Christmas alone and believing "one good turn deserves another". The two friends then returned to Sodor.
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 the sixth season episode, A Bad Day for Harold the Helicopter. Since then, he has acted a lot more mature and almost never teases the engines. He takes his job as part of the Sodor Search and Rescue Centre team very seriously and is always alert when there is an emergency.
Harold is based on a Sikorsky S-55 helicopter (also known as Sikorsky H-19 Chickasaw), built in the UK as the Westland Whirlwind. He has added floats. Around 70 of these still survive in some form.
Harold is painted white with red lining. His name is painted on his sides, near his tail, in red.