"Devious" Diesel is a devious Diesel shunter.
When Diesel came on trial to the North Western Railway, 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, and Diesel blamed Duck for not helping him. Diesel swore a vendetta, 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 boots and tried telling lies about Henry. He was found out and sent home in disgrace.
Several years later, Diesel was brought back to help on the Ffarquhar Branch Line whilst Percy was being repaired. Diesel tried to win Thomas and Toby over, without success, and after he destroyed several trucks he was to be sent away. However, on his day of departure, Clarabel slipped on oily rails left by Daisy and came off at some points near Dryaw. Diesel, realising that Thomas was blocking his only way home, went to the rescue. Clarabel put in a good word for Diesel, and Diesel, although he returned to the Other Railway, is always welcome on Sodor.
Diesel was sent away, but returned when Duck and Percy needed help at Brendam. 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 mysteriously snuck back to give Mavis advice and taunt Gordon about the triumph of diesel power on the mainland. It is possible that these events happened during Diesel's stay, or that Diesel returned at some stage 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 yet again. Amazingly, Diesel was not sent back this time round, and has 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's old and clapped out, Ben about there was a monster inside a tunnel, and Thomas about the "Curse of the Cliffs" and make him scared 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 Crovan's Gate Steamworks. He later teased Flynn for just standing around and doing nothing, but then witnessed Flynn putting out Thomas' firebox at the Dieselworks.
Although unconfirmed, it can be assumed that Diesel is now a permanent resident of Sodor.