Percy was probably built by Avonside and went gone through a subsequent rebuild in the years preceding his second-hand sale to the North Western Railway. He is said to have been found wearing Hunslet parts, according to workers at Crovan's Gate.
When Gordon, James, and Henry went on strike in protest of the extra work they had to do since Thomas left to run his branch line, the Fat Controller decided that the railway needed another tank engine. Going to the engine workshop, the Fat Controller saw a green saddletank and when the engine confirmed he was willing to work hard, the Fat Controller bought him and named him Percy. Bringing him back to Sodor, Percy was charged with running the branch line while Thomas ran the Main Line with Edward and was allowed to stay after the big engines surrendered. At first, Percy liked to annoy the big engines. Percy was later waiting at Tidmouth, but forgot to whistle to the signalman, and thus was startled when Gordon came running towards him with the express. Gordon managed to stop in time, but Percy was so frightened, he ran away, and ended up in a sandbank. He was rescued by Gordon and took up the duty of station pilot at Tidmouth.
Percy was still a cheeky engine and often liked to play jokes on Gordon and James, but one day the other engines got their revenge when Percy failed to recognise a "backing signal". After this, Percy was subject to bullying from the big engines until Duck arrived and helped Percy to put the big engines in their place.
In the seventeenth season, he let Caitlin pull the mail train for him when she was spending the night on Sodor, he was one of the engines who didn't believe at first that Charlie saw an elephant on the line, has been scared by James twice (when James described the phantom express and teased him for being scared of the dark), and helped Bill and Ben when Ben had derailed. He later attended the fireworks display at Ulfstead Castle, slept at Whiff's waste dump while the turntable at Tidmouth Sheds froze, chased Diesel 10 when the diesel stole some garland from the sheds and gave Sidney new wheels as a Christmas present.
As the television series has progressed, Percy has been dumbed-down in order to teach viewers difficult words: he would say "dingy-fried" where anyone else would say "dignified". He is also shown to make silly mistakes, but then fix them upon realizing what he has done before further damage is caused.
Percy is incredibly accident-prone and holds the dubious record of the most accidents in one season - five, in the second season - crashing into some buffers, falling into the sea, being pushed onto a brakevan, colliding with a cart, and being hit by a falling crate of treacle. So far, his silliest accident yet was in the sixth season,when he slipped on oily rails, crashed through the chocolate factory, and emerged covered in chocolate. Despite this, he is always tries to do his best, learn from his mistakes, and get his jobs done--a loyalty which Sir Topham Hatt has praised him for on several occasions
Since the second season, Percy has had a small rivalry with Harold, stemming from their race. However, they are always willing to help each other when in trouble.
He also had held a grudge from the big engines about their tricking him into misinterpreting a signal, but silenced this with some help from Duck. This shows that, when nonsense goes too far, Percy will not stand for it and seek help to settle the matter for good.
Percy is primarily based on an Avonside 0-4-0ST with the body of GWR 1340 Trojan, the bunker of GWR 1361, Hunslet underframes and valve gear, a smokebox protruding from his saddletank, external cylinders and motion, and sandboxes underneath the tanks on his footplate.
Modifications made to Percy's shape include the angled smokebox supports and the cab being cut down slightly from the back for the bunker.
Percy is painted NWR green with NWR red-and-yellow lining and a "6" painted on his bunker sides in yellow.
In the Railway Series, he carries two builder's plates, one on either side of his cab.
The Reverend W. Awdry and C. Reginald Dalby did not get along very well. Awdry considered Dalby's drawings "toy-like"; this relationship was worsened by the creation of Percy, who was depicted as an Avonside 0-4-0ST/GWR 1340 Trojan with a bunker.
When Percy the Small Engine was published, Awdry wrote to Dalby, criticising his illustrations and claiming Percy resembled "a green caterpillar with red stripes". This was too much for Dalby and he promptly resigned. However, the insult was not forgotten: Thomas called Percy "a green caterpillar with red stripes" in Tramway Engines as well as the televised episode Woolly Bear.
It is debated as to what year Percy arrived. The Island of Sodor: Its People, History and Railways states that Percy was made the station pilot at Tidmouth in 1949, somewhat implying that he arrived on Sodor in that year. However, Henry is said to have been rebuilt in 1935 and Percy makes several appearances in the presence of the pre-rebuild Henry, suggesting that either Henry was rebuilt later on or that Percy arrived before 1935. A post on SiF states that Christopher Awdry affirmed the arrival date in The Island of Sodor: Its People, History and Railways to be incorrect and that Percy came to Sodor before 1935.