Diesel

History
Diesel came on trial to the Island of Sodor from the Mainland, where Duck was asked to show him around. Duck was unimpressed by Diesel's claims of being "revolutionary" and left him to his own devices. As a result, Diesel was made into a laughing stock by the trucks for trying to pull some rusty old trucks and failing spectacularly, to which Diesel blamed Duck for not helping him. Diesel swore a vendetta plan and spread rumours that Duck had given the big engines rude nicknames. Duck was sent to Wellsworth pending an inquiry into the matter, but Diesel got too big for his buffers and tried telling lies about Henry. He was found out and sent home in disgrace.

Diesel returned when Duck and Percy needed help at Arlesburgh Harbour. 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 mysteriously reappeared to give Mavis advice, taunt Gordon about the triumph of diesel power and shunt trucks in the Shunting 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 was sent back to the Mainland yet again.

Diesel returned yet again, this time to assist Fergus at the Sodor 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. Diesel stayed on the Island of Sodor permanently.

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 and proved to Thomas that he did not 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 to be really useful.

In Day of the Diesels, Diesel was 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.

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.

Later, Diesel 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 Hiro told Diesel about it, 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 cause trouble for Hiro. To make the trucks stop, Hiro braked suddenly before the platform, causing three barrels of cream to fly out and splash all over Hiro, Edward and the Fat Controller. Diesel tried to get away, but was blocked by Thomas. As punishment, Diesel had to learn the proper way of handling trucks by Hiro.

Diesel, along with Sidney, Den and Dart, hid from Paxton at the Dieselworks, which resulted in Paxton going on a wild goose chase to find all of the diesels and ended with Diesel running out of fuel as he fled from Paxton, 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 Station during the Christmas holidays and suggested that the last train might have to be cancelled. He also teased Samson about how he 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. Diesel later found Percy stuck in a flood with his fire out. He tried to rescue Percy, 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, 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. Diesel was feeling very merry and he brought the Fat Controller to the Earl's Christmas lunch at Ulfstead Castle.

Diesel was shown to care for some ducklings that he thought were cute. Later, after Annie and Clarabel played a trick on Daisy while Thomas was on quarry duty, 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 planned to demonstrate how strong he was by pulling the train, while in reality, Den, Dart and Paxton were helping him. However, his unusual train was blocking any way into Knapford Yard. Thomas was coupled to the train to move them. Den, Dart and Paxton heard 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. Diesel later got stuck inside one of the crates, and was accidentally loaded onto a ship by Cranky.