Daisy is a green diesel railcar who was brought to the Railway after Thomas broke through the stationmaster's house and "visited him" for breakfast.
Daisy is a diesel railcar that was built especially for service on Thomas' Branch Line. When she first arrived, she was highly strung and would only carry passengers because she felt she was too good to pull trucks. Daisy soon learned that all work is important on Sodor, and now helps out whenever she can. 
When Thomas had an accident, Daisy was brought in to help Percy and Toby run the Branch Line. But she proved to be troublesome; stuckup, lazy and stubborn, and only did whatever work she chose. The Fat Controller threatened to send her packing until Percy had his predicament, leaving Toby and Daisy to run the branch by themselves. And after Toby put in a good work for her, she was allowed to stay.
Daisy helps Thomas with the passengers and sometimes takes the milk or looks after Henrietta for Toby. But to be fair, although still flouncy, she's really a sweet engine and takes a liking for Mavis especially.
Daisy used to be lazy, impatient and stubborn and believing that anything up-to-date is always the best, Until she was threatened to be sent away. She's a good sort really, who is kind, sweet and hopeful.
Daisy is painted Brunswick green with yellow lining, similar to the livery of the British Railways DMUs in the 1960s. She wears make-up, including eyelashes, red lipstick, blush, and blue eye-shadow.
Daisy's face is grey and she has white buffers.
- It is rumored that some of Daisy's television series faces are just BoCo's with added makeup and plus, Daisy used BoCo's chassis in the television series.
- Daisy was the first female engine and diesel engine introduced in the TV series, the second being Mavis and the third being Frankie. (Not counting Rusty the Little Diesel.)