Emily is an emerald Stirling Single tender engine who works on the North Western Railway and is the railway's number 12 engine.
She works mostly on the Main Line in odd jobs and mixed traffic duties, and once ran the Misty Valley Branch Line. Following her rescue of Gordon from a collapsing bridge and James, Percy, and Rebecca from a derailment, she was rewarded with being appointed safety engine, gaining her number with it. She also has her own set of coaches given to her because of her bravery saving Oliver.
When Emily was new to Sodor, she was ignored by the other engines for borrowing and pulling Annie and Clarabel without Thomas' permission or knowledge. The matter was resolved after she saved both Thomas, Oliver and Toad from nearly having a nasty accident on the cross-points and she was rewarded with two new coaches. Emily proved to be kind-hearted, as she was the only engine concerned about Henry when he was ill due to leaky tubes, she stuck up for Salty when Thomas and Percy were teasing him and suggested that Bulgy help the farmer deliver his vegetables around the island, after his passenger services went wrong with some naughty hens.
However, Emily has proven to be a nuisance on occasion as she bossed everyone about when they were clearing up after a storm, Thomas when he was taking passengers and Percy when he was shunting trucks. When pulling the express for the first time, Emily was determined to keep to Gordon's timing. Unfortunately, she became impatient and ran out of water.
Later on she was assigned to the Flour Mill. She soon met with James who was assigned to the Misty Valley Branch Line, primarily the Black Loch and he told her that there was a monster in the Loch. She had trouble with the Flour Mill and ended up crashing some trucks off the line. She was later reassigned to the Misty Valley Branch Line replacing James, where she discovered that the monster was just a family of seals. She is presumed to still work the line as it has never been stated otherwise.
When Thomas had to stay at Knapford Sheds, Emily did not like his company at first as she did not want any noise; she just wanted some sleep. However, she learned how comfortable it is to have the company of friends, missing Thomas when he did not return to the sheds. Originally disappointed once Tidmouth Sheds was rebuilt, she was delighted to find that the shed had gained a seventh berth and she was transferred there to be with her new shed mates.