Harold is a helicopter who patrols the Island of Sodor, searching for emergencies.

When Harold was first introduced, he was shown to be very proud of being able to fly and the speed and versatility this gives him. He somewhat tended to be cocky, as he believed that "railways are slow and quite out of date". This caused a bit of rivalry between him and the engines, particularly Percy and he is usually nicknamed "whirlybird" for that, but he usually got his comeuppance in the end. Sometimes, he finds himself in need of Percy's help after he teases him for being slow. Despite this, he still works hard and deep down, he knows that the engines on Sir Topham Hatt's railway, including Percy, are just as useful as he is. He is always helpful and highly alert whenever there is an emergency, especially on patrol. Because of this, he remains a friend to the railway.