Percy is a saddletank who works on Thomas' Branch Line.
Percy was built by Avonside 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, Sir Topham Hatt 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 Knapford, 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.
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 He met a self-centred white helicopter named Harold who said that railways were "slow and out-of-date", so he decided to race him, of which Percy won.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.
One day at the harbour Percy persuaded the troublesome trucks to push him past the danger sign. This resulted in Percy plunging into the sea and having to be sent to the works.
One winter, the cold was severely getting to Percy, so he tried to think about warm things. As a result, his workday led to Percy obsessing over scarves and seeing everyone around him wearing them. Henry remarked on the silliness of an engine wanting a scarf, but Percy bit back with a comment about Henry's funnel size. The cold did not affect Percy's cheekiness however, as he planned on startling the coaches by coming in quietly. However, the porters did not hear him either and Percy crashed into their luggage trolley, ending up covered in jam with the Fat Controller's hat on Percy's lamp iron and his best trousers wrapped around Percy's funnel. James joked that Percy found a scarf after all and ran off to tell Henry.
Percy overheard Mr. Conductor talking in his sleep. Later he told Thomas about Mr. Conductor and concluded that he travels on Sodor to the Magic Railroad. Thomas told Percy about the missing truck and realised that the buffers lead to the Magic Railroad and told Percy to guard the buffers to be a brave engine. Percy found Splatter and Dodge at the magic buffers and raced to warn Thomas, telling him to get Lily back to Muffle Mountain before Diesel 10 destroyed the Magic Railroad.
Thomas told Percy about some mysterious footprints that he saw at the Sodor China Clay Company. Percy convinced himself that it was be footprints of a monster. For the next couple of days Percy was scared of the monster and mistook Gator for a monster. He later learned that the monster was really a dinosaur a long, long time ago, which had now become a fossil. Percy was starting to miss Gator and tried 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 did not stop to be coupled and the Troublesome Trucks went rolling into Ulfstead Mine. He thought about Gator and his advice of how to be brave and went into the mine and rescues the trucks. Percy reunited with Gator at Christmas.
He was to take The Mayor of Vicarstown to the Sodor Search and Rescue Centre to present a new alarm bell to celebrate their one thousandth rescue. Diesel overheard Percy talking about it at Brendam Docks and he decided to trick Percy by sending him 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. Once, he saw some painters at Knapford, talking about the Fat Controller "leaving". He told Thomas and the other engines and they came to the conclusion he was leaving Sodor because of them, so they decided to create no confusion and delay until he decided to stay. By the end of the day, it appeared their plan was successful, though Percy still had his doubts. His suspicions were seemingly confirmed when he saw the painters clearing out the Fat Controller's office. Now believing the Fat Controller was being forced to leave, the engines decided to go on strike until he was allowed to stay. After much confusion and delay, the Fat Controller explained to the engines that he was just moving to a temporary office while his was being redecorated.
Percy had the job of taking the Christmas Mail to Vicarstown for Hiro to take to the Mainland and onwards to the North Pole. Harold offered to help, but Percy refused, determined complete the job on his own. Percy left a bag of letters behind at Dryaw, Harold chased after him, but Percy, believing he was mocking his speed again, refused to stop and crashed into a snowdrift at a tunnel. Stuck, he reluctantly agreed to let Harold deliver the Mail. The following day, he heard from Toby that Harold had not returned to Sodor that night, so he raced off to the Mainland to find him. He found him stuck in a field and out of fuel. Not wanting Harold to spend Christmas stranded and alone, he fetched his fuel and the two returned to Sodor.
Percy took Thomas' place in the shunting competition of The Great Railway Show after Thomas had an accident and was left on Sodor. However, Thomas managed to make it to the Mainland after all and Percy dropped out of the competition so his best friend could compete and because he did not want to compete in the first place.
Percy became confused because of the new P.A. system which was not working properly. Because of this, he ended up taking a delivery of pigs to Lord Callan at Callan Castle. He was also told by Dowager Hatt to pull Gordon's Express instead of Gordon doing so, but because he was not strong enough to pull it, he got stuck on Gordon's Hill and ended up severely delayed.
When Merlin first came to Sodor, Percy thought everyone was talking about how Merlin is an invisible engine so that why Percy hadn't seen him. He looked everywhere for the invisible engine until he met up with Merlin and heard what he believed was the invisible engine's whistle and rushed off to tell Thomas about hearing what he thought was Merlin's whistle; but unknown to Percy, it just turned out to be Trevor.