Harold is a white helicopter that works for the Sodor Search and Rescue Team.
Harold arrived on Sodor many years ago and was used to deliver supplies to Knapford Harbour, which was being built at the time. During that time, he was working closely to Percy the Small Green Engine, who was delivering supplies as well via railway to the Harbour. They maintained a tight relationship since their first meeting, and even had a random race during a time when they were both delivering supplies. Percy arose victorious and has been rather conceited towards Harold ever since.
Harold helped Percy yet again when his mail duties were too much for him to handle alone.
Harold likes to fly by the Skarloey Railway, once gaining yet another rivalry with young diesel, Rusty.
When Butch was broken down in the middle of the road, Harold spotted his comrade and flew off to seek help for him.
When The Flying Scotsman was stranded on the Main Line, Harold flew to the Lumberyard knowing they had the equipment to aid in clearing the rockslide. When he arrived at the Lumberyard, the foreman was quick to hurl insults at the helicopter. He was able to convince Carlo to send Diesel 10 to the site however.
Harold yet again assisted in a time of tragedy by delivering The Thin Controller and many other men during the Munitions incident, but got "choked up" by the casualties caused by the foolish General Zen.
Harold was part of the team that responded to the fire at Last Haven.
Harold was dispatched by the Search and Rescue Centre to rescue a wanderer who was dangling from the bridge over Poll-ny-Chrink. However, the shepherd refuses to grab Harold's rope, instead clinging to a strange object. Upon failing to rescue him, he flies to Shiloh and asks Culdee to attempt a rescue by rail. Upon refusing, Harold departs to report the situation to the Search and Rescue Centre.
Harold is one of the kindest, most compassionate characters to roam Sodor. He loves his life devoted to rescue and helping others, and, despite some "less than amazing" friendships maintained with some other engines, Harold remains as kind and loyal as ever.
- Season 0: Old Groaner (cameo), Snow Problem (does not speak)
- Season 2: Breakdown Blues, Mavis and the Tornado, Rosie, Rock-Star, Munitions, Swashbuckler
- Season 3: Samson, BoCo, and the Man in the Hills (cameo), Culdee Fell
- Harold used to work at Dryaw Airfield near Thomas' Branch Line before being relocated to the Sodor Search and Rescue Centre, hence his arguments with Percy the Small Engine.