- "Sodor engines are all heroes to me!"
- — Belle, Day of the Diesels
When Belle first arrived on Sodor, she helped Thomasand Percy put out a fire at an old farm shed. Belle later advised the Fat Controller to bring Flynn to Sodor. When the Vicarstown Dieselworks was on fire, Belle and Flynn put out the fire just in time.
Belle later tried to befriend Toby when she learned that they both had bells. At first she did not understand that Toby was an older engine that may not like the same things she does, but she soon realised her mistake and they are now firm friends.
Belle attended the welcome party for Sir Robert Norramby and tried to help in the search of Stephen when he went missing.
When some railway inspectors questioned whether the Fat Controller needed two fire engines, she and Flynn became worried. She beat Flynn to two consecutive fires, but tried to comfort him when he subsequently became depressed. Her words did not do much to lift Flynn's spirits. She stayed on guard at the fireworks display at Ulfstead Castle when Flynn refused to do it; when Flynn was called to another fire at Brendam Docks, they both realised that Sodor really does need two fire engines after all.
When a whale was beached at Bluff's Cove, Belle and the rest of the Rescue team came to help put it back in the water. She suggested Toad sing to it while they took it to Brendam to be released back into the ocean. She and the other members of the Rescue Centre attended the team's 1000th rescue and helped out Percy and Diesel at the Old Quarry. After Rocky fell off the tracks at Ulfstead Castle from rescuing Henry, Belle, Flynn, Butch and Harold came to lift him back onto the rails, though they were arguing about who was the best. Thomas snapped their argument and told them that they needed to work together. They all agreed and pulled Rocky back onto the tracks.
One summer, she and Flynn were busy putting out fires caused by the boiling summer weather. They came across a stretch of buckled track, where Flynn suggested that Belle pour water on the rails to prevent further buckling, though Belle decided to keep her remaining water in case they came across another fire. This decision came back to bite her, as she later derailed on the buckled tracks while responding to an emergency. Luckily, Flynn was able to extinguish the fire and Thomas brought Rocky to put her back on the rails.
Belle is a bold, big, blue tank engine, who is fitted with two water cannons atop her tanks, from which she can shoot water to fight fires. She also has a whistle and a big brass bell. Belle is lively, loud and proud, with a big personality. While she may come across as bolshy, highly self-assertive and overwhelming, Belle will always have the best of intentions at heart. She always wants to do her best and be the best she can be, causing her to be daring, energetic and bursting with enthusiasm. But Belle is happy to talk to any engine or vehicle and although she may be quick to jump in with her own ideas, always tries to understand, help and be kind to others. As a result, she is a very likable engine, so much so that even Cranky gets on with her. She gets on well with Thomas and can even hold a civil conversation with Diesel, but she works especially well with Flynn, when helping him to tackle fires and is always there to support him should he make a mistake. Although fighting fires is always at the top of her list of priorities, Belle has also been known to perform goods duties about the railway.
Belle is based on a British Railways Standard Class 4MT tank engine with added water cannons, brass bell and electric headlamps. These tank engines were a development of the LMS Fairburn 2-6-4T and were used for commuter and outer suburban services. They were used on all regions of BR, except the Western Region, and remained in service until 1967. Fifteen of these engines are preserved, three at the Bluebell Railway.
Belle is painted royal blue with red and gold lining and brass fittings. She has the number "6120" painted in gold on her side tanks and a crest consisting of a flame, two dolphins and a crown painted on her bunker sides. Her water cannons and wheels are painted red, with the latter having blue rims.