When Emily was new to Sodor, she was ignored by the other engines for pulling Annie and Clarabel without Thomas' noticing, permission or knowledge. The matter was resolved after she saved both Thomas and Oliver from 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 and when she stuck up for Salty when Thomas and Percy were teasing him.
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 taking passengers and Percy when shunting trucks. When pulling the express for the first time, she was determined to keep to Gordon's timing, only to run out of water.
When Thomas had to stay at Knapford Sheds, Emily did not like his company at first as she just wanted some sleep. However, she learned how comfortable it is to have the company of friends and once Tidmouth Sheds was rebuilt, she was transferred there to be with her new shed mates.
Later on she was assigned to the Flour Mill, while James was assigned to the Misty Valley Branch Line, primarily the Black Loch. James 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. 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.
Like most engines, Emily can get quite puffed up in the smokebox. When she was called "Excellent Emily", she ignored the advice of Murdoch, Thomas and Duck only to get herself into trouble. Afterwards, she took their advice to get her job done.
Emily tries her best to make her friends happy. When she assumed that Mavis was sad, she took flowers to the quarry, but only created a mess. When she tried to help her friends with the football/soccer match, she neglected to fetch the dirty washing, which turned out to be the football/soccer team's uniforms. Also, when the Fat Controller lost his top hat, Emily went to find it without success and then took the special Christmas train without permission. However, she always makes amends for her friends' sake.
When Thomas hid his snowplough, Emily had to do Thomas' work and saved him when he got stuck in the snow again.
Thomas told Emily about the Flatbeds of Fear, but Emily did not believe in it and said there was a sensible explanation. She eventually believed in it, until Thomas told her that it was just one of Salty's tall tales.
When the Steam Team tried to clear the tracks of snow for Connor to deliver the last train for Christmas, Emily was unable to clear them from the Sodor side, but was happy to see Hiro as he cleared it from the other direction, then was disappointed when they mistook Harold for Father Christmas by believing his red port lamp to be Rudolph's nose.
She was later informed by Paxton about the rumour that Toby was to be scrapped, but Henrietta overheard and knew it was not true. Emily took her to find Toby and once they found him safe, Emily gave him some of her coal.
When Henry confided in Emily that he feared catching chicken pox from Stephen and Bridget Hatt, she assured him that he could not get chicken pox, joking that he worries too much.
She became fascinated with Caitlin being able to go at such high speeds and wondered what it would be like to be fast, becoming depressed when she could not do so. However, in the end, she managed to save Caitlin when she descended down a steep hill with broken brakes.
When Thomas was fed up with Diesel's selfishness during the holidays, Emily partook in his plan to mend the diesel's ways, much to her joy and amusement. She later attended the Earl's Christmas lunch with several other engines, including Thomas and a reformed Diesel.
Emily went with the other Sodor engines to be part of The Great Railway Show, competing in the Best Decorated Engine Parade along with James, Carlos, Rajiv and Yong Bao, but she and James lost to Rajiv. After Percy dropped out of the Shunting Challenge, she encouraged Thomas to take his place, declaring him Sodor's last chance to win.
In the seventh season, Emily acts as a sisterly figure to the others. Emily is kind and compassionate, and is always sensitive to the other engines' feelings. If she wrongs someone, knowingly or not, she is quick to apologise.
From the eighth to the sixteenth seasons however, Emily occasionally had tendencies of being rather bossy and testy to others. She strives to be one of the best and sometimes causes mishaps, although she usually makes up for her mistakes when she realises her errors.
From the seventeenth season onwards, Emily has matured and has been portrayed with her original personality, with subtler shades of bossiness or overconfidence. She often acts as a voice of reason to the rest of the Steam Team, particularly when James gets too big for his wheels, or when a friend needs moral support.
Emily is based on a Great Northern Railway G3 Stirling Single 4-2-2. Designed by Patrick Stirling in 1870 with 8ft 1in driving wheels and outside cylinders, they were specifically built for high speed expresses between London and York. These locomotives were capable of hauling 280 ton trains at 50 mph, though they could reach 85 mph on lighter services. A total of 53 were built at Doncaster between 1870 and 1895, in three series introduced in 1870, 1884 and 1894 respectively; the last was withdrawn in 1916. The only surviving member of her class (No. 1) is on display at the National Railway Museum.
Emily is painted emerald green with gold and yellow lining. She has chocolate coloured running-boards and her buffers were originally bronze but are now silver. For the Great Railway Show, gold trim and flowers were added to her coat for her participation in the best decorated engine parade.
Emily is the first character besides Thomas himself to say his infamous phrase, "Cinders and ashes!", which was done in the thirteenth season episode, Tickled Pink.
Some literary articles show her being sisterly mostly to Thomas and James: according to a magazine bio, she has a particular fondness for James and treats him like a little brother. In Railway Rhymes, the author states that Emily and Thomas act like brother and sister.
Emily's seventh season whistle sound would later be re-used for Peter Sam and Freddie.