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When Harold first met Percy, he stated that railways were "slow and out-of-date", but was proven wrong when Percy beat him in a race.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.
Harold used to live at Dryaw Airfield by Thomas' Branch Line. He keeps a watchful eye over Sodor in case of trouble and speeds to the rescue of anyone in distress.
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.
- Percy the Small Engine
- More About Thomas the Tank Engine
- Thomas Comes Home (does not speak)
- Thomas and the Fat Controller's Engines (cameo)
- Thomas and the Missing Christmas Tree (does not speak)
- Season 2 - Percy and Harold, The Runaway, and Thomas and the Missing Christmas Tree (does not speak)
- Season 3 - Percy's Promise, Thomas, Percy and the Post Train, All at Sea, and Thomas and Percy's Christmas Adventure
- Season 4 - Trucks!
- Season 5 - Baa! (does not speak), Toby and the Flood, Thomas and the Rumours (does not speak), Oliver's Find, Sir Topham Hatt's Holiday, and Make Someone Happy
- Season 6 - A Bad Day for Harold the Helicopter and Percy's Chocolate Crunch
- Season 7 - Emily's New Coaches (cameo), Harold and the Flying Horse, Best Dressed Engine (cameo), The Grand Opening (does not speak), and Three Cheers for Thomas
- Season 8 - Don't Tell Thomas
- Season 9 - Thomas and the Rainbow (does not speak), The Magic Lamp, and Bold and Brave
- Season 10 - Topped Off Thomas (does not speak), Thomas and the Birthday Mail, and Thomas and the Treasure
- Season 12 - Henry Gets It Wrong and James Works it Out
- Season 14 - Henry's Health and Safety (cameo), Being Percy (cameo), Jumping Jobi Wood! (does not speak), Jitters and Japes (cameo), and Merry Misty Island
- Season 15 - Toby and Bash (cameo), Edward the Hero, James to the Rescue (cameo), Happy Hiro (cameo), Henry's Happy Coal (cameo), Let it Snow, Stop that Bus!, Stuck on You, and Fiery Flynn (cameo)
- Season 16 - Race to the Rescue (cameo), Express Coming Through (does not speak), Sodor Surprise Day (does not speak), and Welcome Stafford (cameo)
- Season 17 - Not Now, Charlie!, The Thomas Way, and Too Many Fire Engines (does not speak)
- Season 18 - Duck in the Water (does not speak), Thomas and the Emergency Cable (mentioned), and Last Train for Christmas (does not speak)
- Season 19 - Toad and the Whale, Helping Hiro, Two Wheels Good, Wild Water Rescue, The Other Side of the Mountain (does not speak) and Rocky Rescue
He is likely to appear in the twentieth season.
- Thomas and the Magic Railroad
- Calling All Engines!
- The Great Discovery
- Misty Island Rescue
- Day of the Diesels (cameo)
- King of the Railway (cameo)
- The Great Race (fantasy)
- 1988 - Thomas and Harold, Percy and Harold, and The Runaway
- 1989 - Hurricane!, Diesel's Christmas (does not speak) and An Important Visitor (cameo)
- 1990 - Rumours and Speculation (does not speak)
- 1992 - James's Task
- 1996 - Night Lights (does not speak)
- 1998 - Really Useful Rusty!, The Exhibition! (mentioned) and The Beached Whale!
- 1999 - Thomas is Rescued (does not speak) and Harold and Henry
- 2000 - A Special Story About Harold (does not speak), Harold's Race!, and Harold's View
- 2001 - The Eyesight Test (mentioned), A Special Story About Harold, The Kite Contest, and Harold Goes Quiet
- 2002 - Henry (cameo)
- 2003 - False Alarm, Harold's Bad Day, Henry's Dizzy Day, Surprise, Surprise!, Mountain Rescue, Queen of the Road!, Super Heroes! (does not speak), Flying Fish! (cameo), Broken Bridges (cameo), and Cruise Control! (does not speak)
- 2004 - Pony Pick-Up!, Harold's Bad Day, Going for a Swim!, Special Signal, Henry to the Rescue (does not speak), Dirty Percy!, The Grand Opening (does not speak), Holiday Hat (cameo), Dockers' Dinner (cameo), Emily Saves Henry (cameo), and Night Lights (cameo)
- 2005 - Mine Mystery (cameo) and Tidy-Up Time! (cameo)
- 2006 - Monster Mistake (does not speak), Caroline's Cruise, Passenger Plane, and Sparkling Sky
- 2007 - Airship (does not speak), Autumn Leaves (does not speak), Blown About!, The Telescope (does not speak), Rusty's Rescue!, Playful Passengers (cameo), and Henry's Hairy Passenger (cameo)
- 2008 - Snow Trouble!, Mystery Music (cameo), Big is Beautiful, and Percy's Post!
- 2009 - New Delivery (cameo)
- 2010 - Henry Gets It Wrong! (does not speak), Harold Afloat, Misty Island Rescue (cameo), Saving Santa!, Thomas Takes Off!, Thomas and the Rainbow! (does not speak), In a Spin, and Disgusted Diesels (cameo)
- 2011 - Racing on the Rails, Monster Mistake (does not speak), Misty Island Rescue (cameo), Rosie's Rocks, Harold and the Kites, Pictures in the Sky (cameo), Splashdown!, Tidy Thomas (cameo), and Playful Passengers (cameo)
- 2012 - Edward the Hero (does not speak), Operation Rescue, The Fat Controller's Bedtime Story (Issue 639) (does not speak), Making Hiro Happy (cameo), Where's That Bus?, Happy Harold, Carol Concert, Clock Countdown, A Musical Mystery (cameo), Playful Passengers! (cameo), and The New Delivery (cameo)
- 2013 - Hover Bother!, Sodor Surprise Day (does not speak), Henry's Scary Passenger! (cameo), Toby to the Rescue, and All in a Day's Rescue (mentioned)
- 2014 - The Thomas Way (does not speak), Kite Flying (does not speak), William's Wish, and Edward the Helpful Engine (mentioned)
- 2015 - Duck in the Water (speaks in speech bubble only)
Harold also appeared in the magazine stories, Animals, Bright Sight, Come back, Please!, Company for Cranky, Fishy Diesel!, Glider Rider, Harold and the Broken Tree, Lost Luggage!, On Inspection, Percy's Plight, Ride and Slide, Sky High!, Smoke Signals, Smoky Sky, Sodor Goes Pop, Sound of Silence, Steaming Along!, Stone Signal, The Best Band, The Flying Horse, The Great Air Race, The Great Statue Robbery, The Lighthouse Light, and The Trainspotters.