Percy is a small green engine.
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.