Percy is a cheeky green saddletank engine who was brought to Sodor to help run the railway during the big engines' strike. His favourite job is to deliver the Post Train. He works on Thomas' Branch Line as a goods engine.
Percy was most likely built by Avonside sometime around the 1900's and had 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 and returned to work. 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 stuck 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, although Percy still received a reprimand from the Fat Controller for it.
With Duck's arrival, Percy was sent to Thomas' Branch Line to help build Knapford Harbour and has stayed there ever since as a goods engine. He met a self-centred helicopter named Harold who said that railways were "slow and out-of-date", so he decided, to race him, of which he won. When Percy had to take the Sunday School children home, he came across a flooded paddock which swashed away his fire. Thankfully, with a little help from Harold, who had dropped supplies of hot drinks for his crew and the passengers, he was able to get the children home.
Despite being really useful, Percy was prone to having accidents, sometimes for acting naughty, and others for carelessness. He was naughty when he asked the trucks to push him past a danger notice, only to end up in the water until he could be lifted out and taken to the works by Henry. When a diesel railcar named Daisy came to look after Thomas' Branch Line, she constantly kept leaving behind the milk van every time Percy shunted it for her. Fed up with taking the milk for her, Percy offered to take Toby's trucks for him while he took the milk. However, the trucks were unfamiliar with Percy, and when he ordered them about, they decided to seek revenge by pushing him onto a line of stone trucks.
In Tale of the Brave, Thomas tells Percy about some mysterious footprints that he saw at the China Clay Pits. Percy convinces himself that it must be footprints of a monster that is roaming Sodor. Continuing to be anxious for a few days, he causes lots of confusion and delay such as: mistakenly referring to Gator, the new engine as the monster, failing to delivery his mail on-time and running off before The Fat Controller finished talking to him. After talking with his new friend Gator, he understands what being brave is like and later uses this courage to save James from a huge landslide. He later learns that the monster was really a dinosaur a long, long time ago, which had now become a fossil. He also understood that the footprints Thomas saw, were also only fossils left from the dinosaur. Percy later says goodbye to Gator, who has to return to the mainland, but the two friends are certain they will meet again someday.
In the eighteenth season, Percy was starting to miss Gator and tries to forget him by working hard and not think about him. When he had to deliver trucks full of precious sculptures and paintings for a special exhibition, he didn't stop to be coupled and the Troublesome Trucks are rolling to the Old Mine. He thinks about Gator and his advice of how to be brave. Percy goes into the mine and rescues the trucks. Percy reunited with Gator on Christmas.
In the nineteenth season, he was to take the Mayor of Sodor to the Search and Rescue Centre to present a new alarm bell to celebrate their one thousandth rescue. Diesel overheard Percy talking about it at the Docks and he played a trick on him, sending Percy to the Old Quarry which resulted him ending up on a badly flooded railway line and after Diesel unsuccessfully attempted to pull him out of the flood, he too also ended up getting stuck in the water after it badly damaged his generator. The rescue of Diesel and Percy marked the Sodor Search and Rescue Centre Crew's one thousand and first rescue. Later, after he finished his mail run to took longer to complete, he hurried to collect the livestock wagon as he bumped so hard and bashed into a flatbed. Percy collects very important sheep from Farmer McColl as he carefully set off, as he takes on water, a lamb leaps through the hole in the wagon onto the tracks. Every time he stops or slow down, a few sheep escape from his wagon, as Thomas stops on every time to collect the very important sheep that he left behind. Percy arrived at the fair in Maithwaite, the Farmer McColl realise his wagon is empty, he apologises to Farmer McColl as Thomas pulls up behind and Percy thanks Thomas for helping him. Once, he saw a painters at Knapford, they talking about the Fat Controller is leaving Sodor, packing the things away to the temporary office and even a telephone, he hurried returned to the sheds, to tells the engines that the Fat Controller is leaving Sodor and even the telephone. Thomas tells the engines that they will going on strike until the Fat Controller stays on Sodor. As the engines is going on strike, the Fat Controller explained the engines that he not leaving on Sodor.
In the twentieth season, he takes the letters to Vicartown to Hiro to the Mainland for Northpole, Harold tries to stop Percy that they got one more letters of left behind, as he raced to the tunnel, the snow was falling and blocked the tunnel, he ask Harold to take the letters to Hiro for Northpole. Percy found Harold on the Mainland where he got stuck in the snow after the snow was fallen. Then, he brings the fuels to Harold in time for goodwill. Later, he was take the Mail train at night, as he comes across the Watermill, he hears a moaning sound which give him a fright. Percy tells Thomas and Toby that remembering the story of The Three Billy Goats Gruff, thinks it's a troll that live under the bridge.
In The Great Race, Percy takes Thomas's place in The Shunting Competition of The Great Railway Show after Thomas has an accident and is left on Sodor. However, Thomas manages to make it to The Mainland after all and Percy drops out of the competition so his best friend can compete.
Percy is a rather cheeky, but kind engine, and generally behaves well, except when he is getting into mishaps from being over-eager. He loves teasing the others, especially the bigger engines, such as Gordon and James, but is always brought down to earth in due course by anyone who gives him the moral to learn, especially the Fat Controller.
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",or make "Demonstration" into " Dimer-station". He is also shown to make silly mistakes, but then fix them upon realising what he has done before further damage is caused.
Percy is incredibly accident-prone and has many accidents, sometimes due to his own silliness or his tricks backfiring. He 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 brake van, 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 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.
Although their friendship has been turbulent at times, Thomas is generally portrayed as Percy's best friend, he is also good friends with Toby as well.
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, even if he faces a reprimanding from Sir Topham Hatt for it.
Percy is based on a Avonside 0-4-0ST with 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 possibly a coal bunker.
A similar looking engine albeit painted red, lived with Percy at the Workshop where he used to live before arriving on Sodor, in the My Thomas Story Library book, The Fat Controller.