Harold is a helicopter. He lives at the Search and Rescue Center.
- Season 1: Wilbert, Frank, and the Milk
- Season 4: All For One and One For James
- Season 5: New Heights for Harold
- Season 8: A Challenge for the Controller and Frank and the Fib
- Season 10: Norman Invasion (cameo)
- Season 13: It's Good To Be Belle, Toad Gets Towed (cameo) and Ivo Hugh's Day at the Zoo (cameo)
- Season 14: Hiro and the Heat Wave (does not speak), Hugo Takes Flight, Merlin the Magical Engine (cameo) and Flat Stanley (cameo)
- Season 15: The Island of Misfit Engines
- Season 16: Finding Lorry (cameo)
- Season 17: Dustin the Really Useless Engine (does not speak), Sidney Transplant (cameo), Oliver Confusion (cameo), Toby, Or Not Toby (mentioned), Thomas' Crown Affair (cameo) and Rocked by a Hurricane (cameo)
- In the French version of Keekre24's series, Harold is called Herbert.
Harold is based on a Sikorsky S-55 helicopter with added floats, pontoon water built in the UK as the Westland Whirlwind.