When Harold first met Percy, he stated that railways were "slow and out-of-date", but was proven wrong when Percy beat him in a race.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.
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.
- In the second season, Harold's spinning rotors were achieved using a transparent disk.
- During the third and fifth seasons, Harold's rotor blades were flimsy and they bent a bit.
- Harold's television series model did not have moving eyes until the tenth season.
- One of Harold's models is currently on display in Canada at Nitrogen Studios. Another is on display in Japan.
- In season twelve, Harold was the only (non-human) character to be fully CGI-animated.
- In the early seasons, Harold would sometimes be given a searchlight, such as in The Runaway, Thomas and the Missing Christmas Tree, and Thomas, Percy and the Post Train. From the eighth season onward, the light has been added permanently.
- Harold will guest star in Winnie the Pooh Meets Alpha and Omega: The Legend of the Saw Tooth Cave.
- In the television series, Harold has never had eyebrows.
- Harold was called Herbert in the French dub of the Classic Series.
- According to the Sodor Superstars Hall of Fame featured on Percy's Chocolate Crunch and Other Thomas Adventures, Harold has never flown out of Sodor. However, this is now false, as Harold has been seen on Misty Island in later seasons.
- Harold is a best friend of Thunderlane.