Percy is a little green saddle tank engine who was brought to Sodor to help run the railway during Gordon, Henry and James' strike. He is one of the youngest engines on Sodor and he is also Thomas' best friend and is quite cheeky, often getting himself into trouble with his attempts to play tricks on the other engines. Percy's favourite job is to deliver the mail and he works on Thomas's Branch Line as a goods engine.
Percy is the No. 6 saddle tank engine. He is the junior member of the Fat Controller's railway. He is quite content puffing around the yard with no particular desire for adventure in the world outside. Percy is Thomas' best friend and his favourite job is delivering the mail. He is always happy to help and other engines sometimes take advantage of his good nature. 
Percy was built as a GWR saddletank in 1897 when he first came to Sodor and he loved to play tricks on the other engines icluding Thomas. Percy later had to spend the night at the Docks with Salty because the track to Tidmouth Sheds was closed and Thomas was already there and that silly tow truck Mater was up to his old tricks again he pretended to be a ghost to scare Percy. Later, Percy tried to be Sodor's best fire engine but he wasn't cutout for the job. He later turned into Sodor's favourite Superhero `Iron Engine` and was always raring to rescue anyone in danger that made feel really special indeed and foiled Diesel 10's plan to wreck the narrow gauge railway and he was known a hero to all. When Percy was taking a morning chuff, he rolled onto an old bridge and got stuck on the edge and needed help from Edward and fast. Percy later was haunted by Thomas telling him a spooky story about Spencer the Silver Engine. Percy is scared of scary things.
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 1 series - 5, in the 2nd series - 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 6th season, when he slipped on oily rails, crashed through the chocolate factory, and emerged covered in chocolate.
Since the second series, Percy has had a small rivalary with Harold, stemming from their race. However, they are always willing to help each other when in trouble.
Percy is primarily based on an GWR Avonside Class SS 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 include: the angled smokebox supports and the cab being cut down slightly from the back for the bunker.
He carries two builder's plates, one on either side of his cab.
- Michael Angelis (Thomas and the Magic Railroad; original workprints only)
- Linda Ballantyne (Thomas and the Magic Railroad)
- Keith Wickham (UK; Hero of the Rails onwards)
- Martin Sherman (US; Hero of the Rails onwards)
- Percy's large scale model used for the spin-off series, The Pack is currently on display in Japan at Thomas Land.
- Since the 14th series, Percy's whistle budges up and down when he blows it.
- One of Percy's models is currently on display in Canada at Nitrogen Studios.