"Devious" Diesel (or Diesel) is a devious Diesel shunter and the main enemy of Thomas the Tank Engine.
Diesel first arrived on Sodor in the same way he did in the Railway Series and was sent away, but returned when Duck and Percy needed help at Arlesburgh. Diesel proved to be as bad as ever and was sent away for good after shunting some trucks of china clay into the sea, but he somehow mysteriously snuck back to giveMavis advice, taunt Gordon about the triumph of Diesel power on the mainland, and shunt trucks in theShunting Yards. It is possible that these events happened during Diesel's stay or that Diesel returned at some point with goods from the Mainland.
When Henry was derailed by some trucks, Diesel was brought back as a last resort to do his goods work, but his attempt to prove his worth resulted in him falling in a barge, and he was sent back to the mainland yet again.
Later on, Diesel returned yet again, this time to assist Fergus at the Cement Works. Diesel became jealous of Fergus' reputation as the "pride of the Cement Works" and concocted a scheme to make Fergus run away. Diesel was partially successful - he managed to make Fergus abandon his post for a time - but was ultimately foiled. Amazingly, Diesel was not sent back this time round and remained on the North Western Railway.
Despite remaining tricky and devious, Diesel learned how useful steam engines can be when Thomas brought fresh diesel fuel for him and the other diesel engines and James pushed him back to the Repair Yard when he broke down. However, he tried to convince Gordon that he was old and clapped out, Ben that there was a monster inside a tunnel, and Thomas about the "Curse of the Cliffs" and scared him at Halloween.
But Diesel proved to have a good side inside him when he told Emily about his record he made for shunting lots of trucks, and proved to Thomas that he didn't ruin the photograph for the billboard on purpose.
Although Diesel made fun of James when he was painted pink, he tried his best to prove himself to be as useful as the steam engines when he took all of the Jobi wood to the Sodor Search and Rescue Centre - only to put himself in danger requiring Thomas to rescue him. However, Diesel tried again to prove how special he is when he took Thomas and Rosie's loads to market and stole James' pigs so that the children could see them. But it was only when Diesel brought the load of slate for the school that he proved himself Really Useful.
Diesel was also one of the Diesels that took over the Sodor Steamworks. He later teased Flynn for just standing around and doing nothing.
In Blue Mountain Mystery, Paxton told Diesel about Luke and Diesel tried to get Luke sent away from Sodor.
Diesel was annoyed when Percy thought he was the Monster of Brendam and later teased Thomas for being a stinky engine and teased Gordon when he was fitted with a yellow and black Diesel bufferbeam. When he and Percy collected a calliope, he thought Percy would not be able to make it play, but was surprised when he did.
In the seventeenth season, Diesel was one of the diesels that brought the Christmas decorations to Tidmouth Sheds. Later, he was biffing trucks around even when Edward told him to stop. Diesel spilled the milk in the trucks, and when the Fat Controller opened the truck doors, the milk splashed all over his trousers. When Hirotold Diesel, he told Hiro to toughen up with the trucks. But when Hiro would not listen, he coupled himself up on the back of his train and helped the trucks play tricks on Hiro. To make the trucks stop, Hiro brakes suddenly before the platform, causing three barrels of cream to splash all over Hiro, Edward, and the Fat Controller. Diesel tried to get away, but was blocked off by Thomas. As punishment, Diesel has to learn the proper way of handling trucks by Hiro.
In the eighteenth season, Diesel, along with Sidney, Den, and Dart, hid from Paxton at the Dieselworks, which resulted on Paxton going on a wild goose chase to find all of the diesels and ended with Diesel running out of fuel, so Paxton helped him to the next station to get some more. Later, he argued with Thomas over who should be the back engine for a train of trucks heading from Ffarquhar Quarry to Brendam Docks, and when he tried to delay Mavis returning to the quarry, Thomas took the train by himself, which resulted in him having an accident. Diesel even tried to blame Mavis for it, but the Fat Controller was wise to his tricks and he let Thomas work with her instead. He later joined the engines at Knapford during the Christmas Holidays, and agreed with Sir Topham Hatt that the last train might have to be cancelled. He also teased Samson about how the tank engine got lost when he first came to Sodor.
In the nineteenth season, he 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. Diesel later found Percy stuck in a flood with his fire out. He tried to rescue the tank engine but only made the situation worse when the water damaged his generator, so he got stuck too. After the two were rescued by the Search and Rescue team, Diesel was sent to have his engine repaired. He later covered Paxton in a cloud of black smoke from his engine, leaving Paxton's face covered in black spots.
At Christmas time, Diesel was being very unhelpful with the extra work, so Thomas and the other engines made a plan. On the night before Christmas morning, Thomas, Emily, Salty and Paxton pretended to be ghosts to scare Diesel, scaring him to the point where he flew off the line and derailed. After he was recovered the next day, he learned the true meaning of Christmas. On Christmas morning, Diesel was feeling very merry, and he brought the Fat Controller to the Earl's Christmas lunch at Ulfstead Castle.
In the twentieth season, Diesel was shown to care for some ducklings that he thought were cute. He then realised that diesels can be nice. Later, Annie and Clarabel played a trick on Daisy while Thomas was on quarry duty, and after Diesel was sent to pull them instead, much to their dismay.
In The Great Race, Diesel wanted to go to The Great Railway Show, but the Fat Controller was hesitant. To prove himself, Diesel had crates put over Den, Dart and Paxton so that they would resemble trucks and put them in a long train. When the Fat Controller was in the final stages of choosing who would go to the show, Diesel would then demonstrate how strong he was by pulling the train, while in reality, Den, Dart and Paxton would be doing most of the work. However, his unusual train was blocking any way into Knapford Station Yard. Thomas was coupled to the train both so that he could get into the station and so that he could prove himself worthy to go to the show. The diesels hear Thomas's whistle and thought it was Diesel's horn, so they began pushing, much to Diesel's horror. Thomas then collided with Norman at a set of points and lost his left buffer as a result. Diesel later got stuck inside one of the crates, and was accidentally loaded onto a boat by Cranky.
Diesel is, in essence, the average pantomime villain. He is oily, malicious, devious, scheming and full of ideas to get revenge. He views himself and any other diesel of his kind as highly above the steam engines, and believes that all diesels are "revolutionary". However, despite his antics giving diesels on Sodor a bad reputation, which causes most steam engines, particularly Duck and Thomas, to mistrust him, he still wants to prove himself to be a really useful engine. He is also very proud to be the first diesel on Sodor.
- After losing the Jobi Wood and get told off by The Fat Controller in upcoming film, Pooh's Adventures of Thomas & Friends: Misty Island Rescue, he along with Diesel 10 will make their appearances in Thomas' Adventures of Aladdin, to get his revenge on Thomas for ruining his chance to prove to The Fat Controller that diesel engines are far more better than steam engines
- After Diesel 10 and Discord will become reformed characters, Diesel will become the new leader of The Rough Gang.
- In Unicron Returns, Diesel is possessed by Unicron's blood, Dark Energon.
- Diesel is also best friends with Zach Varmitech, Robbie Rotten and Garble since they share similar devious personalities.
- As revealed in Emily and the Special Coaches, Diesel is a record holder for most trucks/cars shunted in a single day.