Diesel, also known as Devious Diesel, is a shunter in Thomas the Tank Engine whose design is based on the BR Class 08. He appears in many seasons of the TV series and in the movies, Calling All Engines, Misty Island Rescue, Day of the Diesels, Blue Mountain Mystery, King of the Railway, and The Great Race.
Unlike many of the show's designated "antagonists", who were merely rude or bullying, Diesel was very much like a more traditional antagonist and was always thinking up ways to cause trouble for Thomas and his friends. He was also known to be Duck's archenemy as well as Thomas'. Diesel likes to play tricks on steam locomotives, which he hates and wants to become obsolete. He likes to talk to the Troublesome Trucks and tell them to do bad things. Although Diesel wants to prove that diesel engines are better than steam engines and made them miserable, on occasion Diesel has been shown to help the steam engines. Despite his oily persona and malicious attitude, Diesel does want to prove he is a really useful engine. It appears Diesel has a good side in him but one would still have to look deep to find it.
When Diesel first arrived, he was "on trial" and Duck had to show him around the island. Diesel told Duck that diesels were revolutionary. Later that day, Diesel tried to pull some trucks out of a siding but was soon made of a laughing stock by the trucks. Furious with Duck, Diesel began spreading mean-spirited rumors and initially succeeded but was soon found out and sent away.
Diesel returned sometime later to help Duck and Percy in the harbour but all he did was bully the two small engines and caused problems for the railway. He shunted some trucks into the sea and was sent away again.
Diesel returned as a last resort when Henry had an accident with some troublesome trucks. He tried to pull a long line of trucks from a siding only to snap the coupling and fly off into a barge. Sir Topham Hatt was furious and sent him away yet again.
Diesel returned to help Fergus at the cement works and told him that Sir Topham Hatt wanted him to work at the smelters yard. Diesel's plan did work but was not sent back this time round. Since then, it has been presumed that Diesel has become a permanent resident of the Sodor railway. Diesel has also been given more duties at the quarry. Diesel pushed Thomas under a hopper but then broke down when he took on some bad fuel and suprisingly thanked Thomas for his good deed. He told Gordon that he would be scrapped when Gordon started to squeak and rattle. But Gordon was repaired soon after.
In Calling All Engines, Diesel, Arry and Bert destroyed the engine shed for the steam engines which caused the steam and Diesel engines to give each other the silent treatment. When Thomas was to shunt a bridge section into place, Diesel told Thomas he was too weak but Thomas proved himself and got the job done. Diesel now furious with Thomas swore revenge on him. Thomas had to collect paint for the bridge but Diesel puropsely banged into Thomas' paint cars which caused a big mess. A huge fight broke out on the railway soon after and Sir Topham Hatt was cross with everyone. Knowing that important jobs were to be done, Diesel, Arry and Bert teamed up with the steam engines and once the jobs were finished, everyone made amends.
Despite the fact that the diesels had made up with the steam engines, Diesel reverted back to his old ways after the movie. Diesel told Thomas about the curse of the cliffs to scare him out of doing an important job and when Diesel had to work with Thomas in taking flower to the bakery on Halloween night, Diesel made spooky noises to scare Thomas but was soon paid back when Thomas covered himself in flour and scared Diesel.
Once Gordon was given his very own special coaches but Diesel stole them out of revenge because no one noticed his good work for being the best diesel shunter on the railway. Emily eventually tracked him down and saw that his engine was breaking down and gave him a new one. Diesel was suprised and apologized to Gordon and Emily soon after for the trouble he caused.
Thomas had to take some stinky cheese across Sodor and tried to avoid Diesel, Arry and Bert because he knew they would tease him about it. After arriving at the docks, Diesel called Thomas a stinky steamie but had to shunt the stinky cheese on the boat.
Diesel was taking part in being placed on a billboard but when the photographer asked him to move foreward, Diesel rolled in front of Thomas by accident. Once Thomas saw this, he thought Diesel did it on purpose and tried to get the photographer to take another picture without Diesel. But Diesel found out about this and confessed that it wasn't his fault and Thomas apologized to him. This is one of the very few times Diesel is accused of something he didn't do. In a later episode of the same season, Diesel challenged Thomas to a race across the quarry going backwards only to cause chaos and work not to get done.
Diesel laughed at James when he was painted pink and stole special logs needed to build the Sodor Search and Rescue Centre, which resulted in Thomas chasing him down. Diesel found himself going towards the edge of an unfinished bridge putting his life in danger. Thomas soon caught up and saved him. Diesel thanked Thomas for his heroic act.
One morning after a storm, Diesel became jealous of the other engines because they all had important jobs to do. Diesel offered to do their jobs to make himself look good but was only useful when he ended up doing his own job.
When a new fire engine named Belle arrived, Thomas took an interest in her which caused Percy to get lonely. Diesel manipulated Percy to come to the Dieselworks and once there, Percy saw Diesel 10 for the first time. Diesel was one of the many diesels who took over the Steamworks but once the Dieselworks was repaired, the diesels apologized for their wrong doings.
Diesel also teased Flynn for standing around doing nothing, told Percy a colliope would be sent for scrap and antagonized Gordon when he needed new buffers.
In Blue Mountain Mystery, a young narrow gauge engine named Luke was hiding and Diesel found out about him. Diesel tried to have him sent away but was reprimanded by The Fat Controller for making false assumptions.
Diesel teased Stephen for being old, biffing and bashing Hiro with some Troublesome Trucks, played a nasty prank on Paxton, tried to frame Thomas and Mavis for an accident with some Troublesome Trucks and teased Samson about how the tank engine got lost when he first came to Sodor.
Diesel tricked Percy into going to an old quarry so that he could do Percy's job of picking up the mayor to take him to a ceremony and get into the front picture of the newspaper and covered Paxton in a cloud of black smoke from his engine, leaving Paxton's face covered in black spots.
In The Great Race, Diesel wanted the Fat Controller to take him to the Great Railway Show instead of Henry so to prove himself, he had crates put over Den, Dart and Paxton so that they would resemble trucks and put them in a long train. Diesel would demonstrate to the Fat Controller how strong he is by pulling the train, while in reality, Den, Dart and Paxton would be doing most of the work. His plan was foiled however when Thomas was coupled up to the train and the diesels hear his whistle and thought it is Diesel's horn, so they began pushing, much to Diesel's horror. Later, Diesel confronted his lackeys about the incident and nearly forced Dart off the Dieselworks turntable, but instead made Dart bump into a button, sending a crate down on him. Diesel later escaped to Brendam Docks and Den, Dart and Paxton continued to hunt him down, only for Cranky to load him onto a ship bound for the mainland by accident.