Rosie is a small red (formerly pink) tank engine that works on Thomas' Branch Line.
Rosie came to Sodor to help out with Shunting Detail in the hills by Thomas' Branch Line where Thomas would often take Annie and Clarabel. Rosie liked to talk to Thomas, which would annoy him while he was busy taking his Local Train. However, one day when Thomas was taking a special train past the Shunting Yards he was caught in a mudslide due to the rain. Rosie was sent to his rescue and since then Thomas has been her firm friend since. However, Rosie still idolized Thomas after this excursion.
Rosie failed to save Daisy's milk train by breaking the chain. After this, The Fat Controller sent Rosie to the Skarloey Railway Transfer Yards to help out and get away from the Branch Line where she is living in Thomas' shadow. She also worked at The Lumberyard under Carlo Debris at this time. At the Transfer Yards, she saved Rusty from a terrible accident on the bridge and since then she realized that she was special, too.
After hearing about Culdee's accident, she grew concerned for the mountain engine's well-being. After hearing from Victor about how Culdee would cry in the night during his repairs, she eventually found Percy, and asked for his assistance in helping Culdee.
Later, Rosie has returned to the Lumberyard following her good deed when Hank comes by to commend her for her kindness in helping Culdee. When she expresses hesitation to take credit, Hank tells her to give herself more credit for her courageous effort and big heart, and not to let him call her "little lady" ever again!
Rosie is constantly cheerful and is fascinated by her surroundings. In addition to idolizing Thomas, she tries to copy his every move to be just like him. She knows now, though, that when she is just being herself, she is a Really Useful Engine too!
During her stay at the Transfer Yards, Rosie befriended Sir Handel. The two can talk with one another for many hours without realizing how much time has gone by!
Hank acts as a father figure to Rosie, sharing an American heritage.
Rosie is quite empathetic, which became evident in her concern for Culdee's mental well-being after his nasty accident. She does express some meekness however, as she finds it hard to credit her own courage and heart for alleviating the situation.
- Season 1: Lift Bridge, Wilbert the Lumberjack (cameo), Rendezvous with Disaster (cameo), Will Power (cameo)
- Season 2: Serendipity (cameo), Mavis and the Tornado (cameo), Rosie, James Goes on a Trip (cameo), Rock-Star, Munitions (cameo), Swan Dive (cameo), Thomas and the Fortune Teller
- Season 3: Perhaps He's Got a Corset, Samson, BoCo, and the Man in the Hills (cameo), Culdee Fell, Derek and the 2 Faced Engines, Penn Pals (cameo), Ice Breaker (cameo), Percy and Asbestos (cameo), James Goes On A Streak
- Web Clips: Mike and Frank Banter: SLOTLT, ScarletFire, and EE93's Return (cameo), Percy's Kafka Dream (cameo)
- Rosie's model with the new red livery was made by Annie C.