Daisy is a diesel railcar who originally worked on Thomas' Branch Line, following her arrival after the events of Thomas Comes to Breakfast. She now works on the Harwick Branch Line carrying out passenger duties, while Ryan runs the goods duties.
Daisy was introduced in the second season in the same way as the Railway Series. After being confronted by the Fat Controller, Daisy had a change of heart. However, Daisy regained her original attitude later in the fourth season when she ignored Toby's advice about bulls. Daisy has also been seen on the other railway as a unfamiliar diesel. In Calling All Engines! she was one of the diesels who joined the meeting at the coaling plant and later helped Toby take workmen to repair the Sodor Airport.
Starting in Sodor's Legend of the Lost Treasure, she runs on the Harwick Branch Line with Ryan, managing the passenger trains while he handles the goods, although she can still help out on Thomas' branch line if passenger duties aren't too busy on her line.
During the twentieth season, Daisy encouraged Ryan to help her out with some of her passenger duties, but her laziness backfired when Ryan took on even more of her responsibilities and ended up causing confusion and delay. Daisy apologised and agreed to do Ryan's extra duties for a while: taking trucks to and from the Quarry.
The Fat Controller asked Daisy to take some special passengers to the holiday town of Harwick, but she decided that they were so special that she doesn't want to allow children eating ice lollies on board and refused to pull a dusty baggage truck. Without props or an audience, the puppeteers' Punch and Judy puppet show could not go ahead. With no time to go back to the station to get the props, the puppeteers put on their performance from one of Daisy's windows.
Later, Daisy was rude to Annie and Clarabel when she temporarily returned to Thomas' Branch Line. The coaches paid her back by convincing her she was falling apart, when there was actually nothing wrong with her. However, their plan backfired as Daisy panicked and abandoned them with their passengers at Maithwaite so she could speak to a professional. She returned to Harwick soon after, and since Thomas was still in quarry duty at the time, Annie and Clarabel were left with an even worse substitute: Diesel.
In the twenty-first season, Daisy gets upset when Christmas doesn't go as planned. Diesel damages Daisy's springs when he bumps into her and later, thinking that she's asleep, explains that it's his way to make friends, and tells her that Harvey is boasting about being unique, which causes trouble between the two engines.
Daisy is a diesel rail-car, who has been known to consider herself "highly sprung and right up to date". She was brought to help run passenger services on Thomas' Branch Line, while Thomas was "indisposed" after running into the stationmaster's house. She can be hard to please, mostly indignant, and highly opinionated. She is extremely authoritative, trustworthy, and confident in herself and can sometimes boast of her abilities in particular situations, unconcerned, despite having never actually been in such a position before. She will sometimes presume she can show other engines a thing or two, despite knowing little on the matter, until she actually learns for herself.
After arriving on Sodor, Daisy insisted that she required delicate handling and considered pulling trucks and vans to be below her. She was very lazy and stubborn, but the Fat Controller spoke severely to her, telling her that his engines must work hard and that he sends lazy engines away. However, working hard after Percy got himself into a predicament, the Fat Controller felt she deserved another chance. Daisy is now good friends with the other engines on Thomas' Branch Line, tries to work much harder to be a useful engine and is better capable of handling things alone. Daisy can be described as a self-obsessed diva.
Daisy is based on the BR/Metro-Cammell Class 101/102 DMU. She is, however, a single railcar instead of the traditional two to four-piece units. These were one of the longest serving class of "First Generation" DMUs, with its service life of 47 years only being replaced by the Class 121. A total of 41 cars are preserved.
In the Railway Series, Daisy is painted BR Brunswick green with yellow lining with the 'Cycling Lion' British Railways crest, similar to the livery of British Railways DMUs in the 1960s. In the television series she is painted a much lighter green. She wears makeup, including eyelashes, red lipstick, blush and blue eyeshadow.
In the Railway Series, Daisy's face is the same colour as her livery; in the television series, it is light grey like all other engines.