In 1949, Percy ran Thomas' 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 frightened, ran away, and ended up in a sandbank. He was rescued by Gordon, and took up the duty of station pilot at Tidmouth.
In 1955, 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.
With Duck's arrival, Percy was sent to Knapford to help build the harbour, and has stayed there ever since as a goods engine.
In 1987, he met with Albert the tank engine bought from the National Railway Museum, and soon befriended him in the end. Starting 1988, he took Annie and Clarabel whenever Thomas was ill or away.
In 2004, Percy acquired his own coaches and named them Danielle and Fiona.
From Tony Grigg's notes on “The Island of Sodor”:
Number 6 “Percy”
Percy is an Avonside 0-4-0ST of unknown origin, but is almost certainly second hand, with “Hunslet parts”. Percy arrived o Sodor in 1949 and was first used as Tidmouth Pilot, then to help rebuild Knapford Harbour. Percy is now based at Ffarquhar.
Percy is a cheeky tank engine, and is almost always Thomas' best friend. He also is shown to make obvious mistakes. Percy has a slight crush on Mavis.
Percy cares a lot about the other engines and will help them when they need him.
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 primarily based on an Avonside 0-4-0ST with the body of a GWR 1340 Trojan, the bunker of a GWR 1361, Hunslet underframes and valve gear, a smokebox protruding from his saddletank, external cylinders and motion and sandboxes underneath the tanks on the footplate.
Modifications made to Percy's shape are needed 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, letters "NWR" painted on his saddle tank sides in yellow and a "6" painted on his bunker sides in yellow. He carries his number in gold figure on his bufferbeams.