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.
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 also is shown to make obvious mistakes.
Percy is incredibly accident-prone, and holds the dubious record of the most accidents in one season - five, in thesecond 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.
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 "toylike"; 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 criticizing 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.